Canonical model · STRICT ingress body

Canonical Invoice Schema

The exact JSON shape an ERP sends, one record at a time, in the invoice_data_json array of POST /api/ingest. Map your data to these fields on your side and call us. There is no per-caller transformation on the canonical leg — you send this schema, unchanged.

You send the invoice content. Pronalytics supplies the seller party and mints the IRN and QR on accept. Money is a JSON number; dates are YYYY-MM-DD; field names are snake_case; enum input is case-insensitive.

v1 · Canonical POST /api/ingest STRICT leg · full validation FIRS / NRS UBL mapping

One field, one place

Every field below carries a stable id. Hover a field row and click # to copy a deep link straight to it. Where a rule is stated once (the VAT model, the tax-category layers, the payment enum) it is stated only once — cross-references point back to that single definition rather than restating it.

1What you send vs. what we mint#

The canonical model is the customer-ingress body only. Sending a seller-owned or minted field is not an error — it is ignored.

You send	Pronalytics supplies / mints
Invoice content: `transaction_id`, dates, `invoice_kind`, the buyer (non-B2C), `line_items`, tax, totals, references.	The seller party (`accounting_supplier_party`) from your registration; the `irn` and `qr_code`; `business_id`; `service_id`. Never send these.

Hard requirements on the STRICT leg: transaction_id, issue_date, and line_items (at least one), plus full per-field validation. invoice_kind is inferred when omitted (see invoice_kind). Everything else is optional or conditional.

The IRN is derived, not sent: IRN = {SEED}-{SERVICE_ID}-{YYYYMMDD}, where SEED is the uppercased invoice_number (else transaction_id) stripped to A–Z 0–9. Every other character — including / and - — is removed, so INV/261 becomes INV261. Uniqueness is per (tenant, IRN); first-seen date wins.

2Root invoice record#

One Invoice object. Grouped as identity, dates, currency/mode, status, notes, references, parties, lines, and totals.

Identity

transaction_idstring Mandatory #

Your unique reference and idempotency key; it seeds the IRN when no invoice_number is present. It is also the only hard requirement on the LENIENT leg. A unique transaction_id is what guarantees a unique IRN — the seed-stripping is many-to-one.

Maps to Pronalytics-only — seeds the IRN, not a FIRS wire field.

invoice_numberstring Optional #

Human invoice number. Preferred IRN seed; falls back to transaction_id when absent (see §1 for the strip rule).

Maps to feeds irn (minted by Pronalytics)

invoice_type_codeenum Optional default 381 #

FIRS invoice-type code. Defaults to 381 (Commercial Invoice). Full list — credit note 380, debit note 384, and the rest — in Enumerations §10.

Maps to invoice_type_code

invoice_kindenum · B2B|B2C|B2G|G2B Inferred if omitted #

NRS transaction kind. If omitted, it is inferred: any buyer tax identity present (TIN / Tax ID / RC) ⇒ B2B, else B2C. An explicit business kind (B2B / B2G / G2B) is honored unconditionally — accepted even with an incomplete or absent buyer (FIRS clearance is the completeness gate). Explicit B2C must omit the customer party (consumer-PII guard — a B2C carrying a buyer is a hard reject). See rule §11. Uppercased on the wire.

Maps to invoice_kind

transaction_typestring Optional #

A data-mapping helper describing the business purpose of the invoice (e.g. loan_fees, interest_fees, late_penalty). Tenant-defined open string. Not a FIRS field and not the B2B/B2C kind (that is invoice_kind).

Maps to Pronalytics-only — data-mapping helper.

Dates & lifecycle

Field	Type	Req	Notes
issue_date	date	req	`YYYY-MM-DD`. FIRS-mandatory; not future-dated. On the STRICT leg a present-but-unparseable value is rejected (422).
issue_time	string	o	`HH:MM:SS`, 24-hour (FIRS-optional).
due_date	date	o	Payment deadline; defaults to `issue_date`.
tax_point_date	date	o	Tax point if it differs from the issue date.
actual_delivery_date	date	o	Date goods/services were delivered (matters for goods sellers).
invoice_delivery_period	object	o	Service/delivery window `{ start_date, end_date }`.

Currency & mode

Field	Type	Req	Notes
currency	string	o	ISO 4217 document currency. Default `NGN`.
tax_currency_code	string	o	Optional input; defaults to `NGN` (always populated).
vat_mode	enum	o	`EXCLUSIVE` (default) or `INCLUSIVE` — declares whether stated amounts include VAT. See §5.

Status & notes

Field	Type	Req	Notes
payment_status	enum	o	On the invoice record this is the fiscal status `PENDING` (default) / `PAID` / `PARTIAL`. The full six-value lifecycle enum used to update a payment lives in §9.
note	string	o	FIRS note 1 — free-form (stored encrypted at rest).
payment_terms_note	string	o	FIRS note 2 — payment terms (stored encrypted at rest).

Conveniences

Field	Type	Req	Notes
branch	string	o	Issuing office branch. Pronalytics convenience — not a FIRS field (FIRS encodes branch in the TIN suffix).
accounting_cost	string	o	Cost-centre / accounting category.
order_reference	string	o	Purchase-order number.
buyer_reference	string	o	Buyer's own reference for the invoice.

Parties (§3), the document-reference family (§7), line items (§4) and the totals (§8) each have their own section below.

3Party — buyer & other parties#

The customer (buyer) and every optional party share one Party shape. The seller party is never sent — Pronalytics supplies it.

customerParty Conditional #

Required for B2B / B2G / G2B; omit for B2C. For a B2B buyer, FIRS clearance additionally requires postal_address.postal_zone — lga/state codes do not substitute. See rule §11.

Maps to accounting_customer_party

Party fields

At least one identity form is required; everything else is optional. The old JTB tin is the wire id used for clearance and transmit routing.

Field	Type	Req	Notes
party_name	string	req	Legal name. Friendly input key `name` also accepted.
tin	string	1 of 3	Old JTB TIN `NNNNNNNN-NNNN`. The wire id (clearance + transmit).
tax_id	string	1 of 3	New NRS Tax ID (12–13 digit). Carried alongside `tin`.
rc_number	string	1 of 3	CAC RC number. Third accepted identity form.
email	string	o	AP email. Validated as an email address.
telephone	string	o	Must start with `+` (country code) — FIRS rejects otherwise.
business_description	string	o	If present, at least 5 characters (FIRS).
postal_address	PostalAddress	o	Buyer address (friendly input key `address`). See below.

PostalAddress

Field	Type	Req	Notes
street_name	string	o	Street line. Friendly input key `street`.
city_name	string	o	City. Friendly input key `city`.
postal_zone	string	cond	Postcode. Optional for B2C/no-party, but FIRS requires it on a B2B buyer for clearance — lga/state codes do not substitute. Any plausible area postcode clears.
lga	string	o	LGA code (e.g. `70` / `NG-AB-ASO`).
state	string	o	State code (e.g. `4` / `NG-LA`).
country	string	o	ISO 3166-1 alpha-2. Default `NG`.

Other parties

All optional; each takes the same Party shape above.

Field	Purpose
payee_party	Payment recipient if different from the supplier.
bill_party	Bill-to party if different from the buyer.
ship_party	Ship-to party if different from the buyer.
tax_representative_party	Tax agent acting for the seller.

4Line items#

line_items is an array of at least one LineItem. Each line carries its own classification pair and its own VAT treatment — VAT is per-line (see §5).

descriptionstring Mandatory #

What was sold. Becomes the item name and description (e.g. "Portland cement"). The unit of measure is a separate coded field — see unit_code below.

quantitynumber > 0 Mandatory #

The true count of units invoiced, greater than zero. The unit those are counted in is the separate coded unit_code field below.

unit_pricenumber ≥ 0 Mandatory #

Unit price magnitude only (net when vat_mode=EXCLUSIVE). For a credit/debit note, express the reversal via invoice_type_code 380/384 plus absolute amounts — never a negative price (rule §11).

unit_codestring · coded Optional default C62 ("each") #

The unit of measure for this line's quantity — a coded FIRS invoice-quantity-code (UN/ECE Rec 20), never free text. FIRS requires it and its own sample sends a code (e.g. "XBG" = bag).

Leave it blank and it defaults to C62 ("one / each") — correct for most service, subscription and fee lines. Send the code directly, or a friendly word we map for you: EACH→C62, MONTH→MON, HOUR→HUR, DAY→DAY, YEAR→ANN, KG→KGM, LITRE→LTR, BOX→XBX, BAG→XBG, PACK→XPK. Friendly input keys unit_code, price_unit, uom, unit_of_measure are all accepted.

A real unit we cannot map is HELD — we never guess a code (that once caused a FIRS rejection). If you see "not a FIRS invoice-quantity-code", send a valid code or a known word above, or just leave it blank for C62.

line_extension_amountnumber Optional #

Line net. Computed from quantity × unit_price (less any discount) when absent.

line_item_typeenum · GOODS|SERVICE Optional default GOODS #

Picks the classification pair. GOODS uses hsn_code + product_category; SERVICE uses isic_code + service_category (rule §11).

Classification pair

Field	Type	Used for	Notes
hsn_code	string	GOODS	HS code, pattern `NNNN.NN` (e.g. `8471.30`).
product_category	string	GOODS	Goods category (at least 2 chars). Pairs with `hsn_code`.
isic_code	string	SERVICE	ISIC code, pattern `NNNN` (4-digit, no decimal).
service_category	string	SERVICE	Service category (at least 2 chars). Pairs with `isic_code`.

Unknown hsn_code/isic_code are accepted with a warning while the shipped reference lists are seed subsets — the warning rides the feedback webhook, never a reject on our own incomplete data. When a full official list is loaded, unknowns become hard rejects with no change to your integration (rule §11).

Per-line VAT & adjustments

Field	Type	Req	Notes
vat_rate	number	o	Per-line VAT %. Default `7.5`. Forced to 0 for zero-rate categories (see §5).
vat_type	enum	o	Per-line VAT treatment. Default `STANDARD`. Full driver set + FIRS codes in §6. Legacy input key `tax_category_id` also accepted.
discount_rate	number	o	Per-line discount %.
discount_amount	number	o	Per-line discount value. May not exceed line gross (`unit_price × quantity`).
fee_rate	number	o	Per-line fee % (FIRS-optional).
fee_amount	number	o	Per-line fee value (FIRS-optional).
sellers_item_identification	string	o	Seller's SKU / item code.
customer_sku	string	o	Customer's own SKU.

5The VAT model (per-line)#

The single VAT rule for the whole schema, stated once here. Everything else references back to it.

One VAT rule

VAT is computed per line, driven by each line's vat_type + vat_rate. EXEMPT and ZERO lines contribute 0 VAT regardless of vat_mode. vat_mode only declares whether stated amounts are tax-inclusive or tax-exclusive. There is no unconditional divide-by-1.075, and there is no invoice-level fee-amount VAT source.

How each mode computes

vat_mode	Meaning	Per-line math
EXCLUSIVE (default)	`unit_price` is net; VAT is added on top.	`net = qty × unit_price`; `vat = net × rate/100`.
INCLUSIVE	Stated amount already includes VAT.	`net = charged / (1 + rate/100)`; `vat = charged − net`.

How each category rates

Category	Rate applied	VAT contributed
`STANDARD`	`vat_rate` (default 7.5%)	computed
`REDUCED` / `LOCAL_SALES`	the `vat_rate` you give	computed
`EXEMPT`	forced 0	0
`ZERO`	forced 0	0
`WHT`	forced 0 (own subtotal)	0

Caller-supplied VAT. A header vat_amount is honored verbatim when it cannot be computed from the payload (e.g. an out-of-band commission VAT). See §8.

Withholding tax is informational: it is emitted as its own WITHHOLDING_TAX subtotal and does not change payable_amount — it reduces only the cash actually remitted to the seller. A WHT-category line's amount is the withholding base; WHT = base × 7.5% (rule §11).

6Tax categories — 27 tags, 6 drivers#

Two layers, not a contradiction. Layer 1 is the pinned FIRS master list of 27 tax categories — all 27 are valid tags. Layer 2 is the 6 VAT-family codes that actually drive Nigerian VAT computation.

Layer 1 — the 27-category FIRS master list

FIRS pins a master list of 27 tax categories (GST variants, excise, duty, stamp duty, and more). Every one of the 27 is a valid tag. Pronalytics serves the live list, verbatim and cached, from the public reference resource GET /api/firs-resources/tax-categories so you can render a picker. On the wire a subtotal's category is tax_subtotal[].tax_category = { id, percent }.

Layer 2 — the 6 VAT-family drivers

These six codes are what the serializer computes and emits for Nigerian VAT. Supply any of them as vat_type — the friendly name or the FIRS code.

vat_type (friendly)	FIRS wire code	Meaning	Rate
STANDARD	STANDARD_VAT	Nigeria standard VAT	7.5%
ZERO	ZERO_VAT	Zero-rated (e.g. exports)	0
REDUCED	REDUCED_VAT	Reduced rate	rate given
EXEMPT	EXEMPTED	VAT-exempt supply	0
WHT	WITHHOLDING_TAX	Withholding tax (informational)	0
LOCAL_SALES	LOCAL_SALES_TAX	Local sales tax	rate given

Accepted input spellings

Each driver is accepted as its friendly name (STANDARD), its FIRS code (STANDARD_VAT), either in any case (standard), and via the legacy key tax_category_id. All resolve to the six drivers above. A subtotal on the wire carries tax_category:{ id, percent }; the six drivers are the only categories the canonical serializer computes today — a value outside the six is not silently mapped. If you need a category from the wider 27, raise it with Pronalytics so it can be added to the mapping.

TaxSubtotal shape

{ taxable_amount, tax_amount, tax_category: { id, percent } }. percent is the rate (7.5 for STANDARD). Zero-rate drivers carry percent = 0. WHT is a subtotal with the WITHHOLDING_TAX category.

7Document-reference family#

Every reference here is a DocumentReference object { irn, issue_date } (or a list of them where noted). All optional; each links the invoice to a related prior document for the audit trail.

Correction pointers (credit / debit notes)

Field	Type	Notes
reference_invoice_number	string	Original IRN / number a `380`/`384` corrects. Feeds `billing_reference`.
reference_invoice_date	date	Issue date of the referenced original.
billing_reference	array<DocRef>	Explicit prior-doc links. Built from the two fields above when not given explicitly.

Related-document pointers

Field	Type	Links to
dispatch_document_reference	DocRef	Despatch advice.
receipt_document_reference	DocRef	Receipt advice.
originator_document_reference	DocRef	Originating document.
contract_document_reference	DocRef	Governing contract.
additional_document_reference	array<DocRef>	Any other related documents.

DocumentReference shape: { "irn": string, "issue_date": "YYYY-MM-DD" }. A credit/debit note (380/384) normally carries one of these back to the original; a standalone note with no reference is allowed for aggregate adjustments (rule §11).

8Tax totals & monetary totals#

All optional overrides — omit them and we compute from the lines. Supply them to pin an exact breakdown (e.g. an out-of-band VAT figure not computable from the payload).

Convenience header amounts

Field	Type	Notes
vat_amount	number	Computed per-line if omitted. Supplying it overrides the computed VAT (used for out-of-band figures).
wht_amount	number	Serialized as a `WITHHOLDING_TAX` subtotal. Informational — does not reduce `payable_amount`.
total_amount	number	Approximately `payable_amount`. Computed if omitted.

tax_total · array of TaxTotal

tax_totalarray · TaxTotal Optional #

Explicit tax breakdown; computed from the lines when absent. A supplied tax_total is honored as-is, but any WHT still rides through as its own subtotal.

TaxTotal = { tax_amount, tax_subtotal: [ TaxSubtotal ] }, where each TaxSubtotal is { taxable_amount, tax_amount, tax_category:{id,percent} } (see §6).

legal_monetary_total · object

legal_monetary_totalobject Optional #

Explicit totals; computed when absent.

{ line_extension_amount, tax_exclusive_amount, tax_inclusive_amount, payable_amount }. WHT is excluded from payable_amount.

9Payment status, means & charges#

Two distinct things share the name "payment status" — keep them apart.

One payment enum — six values

The canonical payment-status enum used to update an invoice (POST /api/payment/update) has exactly six values, validated identically on every payment surface. Case-insensitive on the wire, echoed UPPER. An unrecognized value is a validation failure.

PENDINGPARTIALPAID OVERDUECANCELLEDREFUNDED

The payment_status field on the invoice record itself is the narrower fiscal status set at creation — PENDING (default) / PAID / PARTIAL. It records where the invoice sits at issue. The six-value enum above is the vocabulary for later lifecycle updates to that payment. They are not the same list; do not conflate them.

payment_meansarray · PaymentMeans Optional #

Each entry { payment_means_code, payment_due_date? }. Codes in Enumerations §10.

Maps to payment_means

allowance_chargearray · AllowanceCharge Optional #

Each entry { charge_indicator (bool), amount, reason? }. charge_indicator true = charge, false = allowance. A document-level charge raises, an allowance lowers, the standard-rated base. Freight is a charge here, never a line.

Maps to allowance_charge

10Enumerations#

FIRS / NRS resource-backed value sets. Tax categories are in §6; the payment-update enum is in §9.

invoice_kind — NRS transaction kind

B2BB2CB2GG2B

invoice_type_code — FIRS invoice-types

Code	Type	Code	Type
380	Credit Note	394	Self-Billed Invoice
381	Commercial Invoice (default)	395	Credit Note Request
384	Debit Note	396	Invoice Request
385	Self-Billed Invoice	397	Final Settlement
386	Factored Invoice	399	Bill of Lading
388	Statement of Account	400	Waybill
389	Purchase Order	402	Shipping Instructions
390	Proforma Invoice	404	Certificate of Origin
392	Consignment Invoice	406	Customs Declaration
393	Self-Billed Credit Note	408	Packing List

payment_status — invoice fiscal status

PENDING · defaultPAIDPARTIAL

This is the status on the record. The six-value lifecycle enum for updating a payment is in §9.

vat_mode

EXCLUSIVE · defaultINCLUSIVE

line_item_type

GOODS · defaultSERVICE

vat_type — the 6 drivers

STANDARD · defaultREDUCEDLOCAL_SALES ZEROEXEMPTWHT

Green drivers rate to 0. Full mapping and the 27-tag master list in §6.

payment_means_code — FIRS payment-means

Code	Means	Code	Means
10	Cash	31	Debit Transfer
20	Cheque	42	ACH Credit
30	Credit Transfer	43	ACH Debit

payment_means_code is a free string on the model; the codes above are the common FIRS values. The full list is served by GET /api/firs-resources.

Reference resources

Public, no-auth, cached 24h: GET /api/firs-resources/{name} for states, lgas, countries, tax-categories (the 27-tag master list), hs-codes, service-codes.

11Validation rules#

Baked into the canonical model. These explain most accept/reject outcomes — read them before you map.

Kind-aware buyer party #

invoice_kind ∈ {B2B, B2G, G2B} ⇒ customer expected (identity + email + address, with postal_zone for clearance). An explicit business kind with no/partial buyer is accepted at ingress and flagged on the feedback webhook — it will not clear at FIRS until the buyer is complete. B2C ⇒ customer must be omitted (no consumer PII); an explicit B2C carrying a buyer is a hard reject. Omitting invoice_kind infers it from buyer identity.

One classification pair per line #

Goods = hsn_code + product_category; service = isic_code + service_category; matching line_item_type. A discount_amount may not exceed the line gross.

Credit / debit notes #

Use invoice_type_code 380 / 384 with positive (abs) amounts; express the reversal via the type plus reference_invoice_number / reference_invoice_date. Never send a negative unit_price. A standalone 380 (no reference) is allowed for aggregate adjustments.

vat_mode net-backout #

EXCLUSIVE (default): unit_price is net, VAT added on top. INCLUSIVE: the amount already includes VAT, so net is backed out as charged / (1 + rate/100) (never × 0.925) and tax = charged − net. Full model in §5.

Withholding tax #

Mirrors FIRS via a WITHHOLDING_TAX subtotal plus the wht_amount convenience. A WHT-category line's amount is treated as the withholding base (WHT = base × 7.5%), not as tax on the full line. WHT does not reduce payable_amount.

Identity precedence #

The old JTB tin goes on the wire (clearance + transmit routing); tax_id and rc_number are carried alongside. A buyer with only a Tax ID or RC clears at FIRS but will not transmit until an old-format TIN is supplied.

HS / ISIC value-set warnings #

hsn_code / isic_code are checked against the loaded reference set as well as the format. While the shipped lists are seed subsets, an unknown code is accepted with a warning on the feedback webhook — never hard-rejected on our own incomplete data. When a full official list is loaded, unknowns become rejects with no change to your integration.

issue_date on the STRICT leg #

On the canonical (STRICT) leg a present-but-unparseable issue_date is rejected (422) — it never silently falls back to today. It must be a real YYYY-MM-DD and not future-dated.

12Worked examples — Grollo & Mollo#

Fictional. Seller Grollo Consulting (supplied by Pronalytics from registration — never sent); buyer Mollo Industries. Amounts in NGN.

A · Minimal B2C — no buyer, one goods line

EXCLUSIVE · STANDARD 7.5% · buyer omitted

{
  "transaction_id": "POS-000817",
  "invoice_kind": "B2C",
  "issue_date": "2026-07-08",
  "line_items": [
    {
      "description": "Engine oil, 5 L can",
      "quantity": 2,
      "unit_price": 12000,
      "line_item_type": "GOODS",
      "hsn_code": "2710.19",
      "product_category": "Lubricants",
      "vat_type": "STANDARD",
      "vat_rate": 7.5
    }
  ]
}

Line	Net	Rate	VAT	Gross
Engine oil ×2	24,000.00	7.5%	1,800.00	25,800.00
payable_amount	24,000.00	—	1,800.00	25,800.00

B · Full B2B — complete buyer, service line, mixed VAT

explicit B2B · one STANDARD line + one EXEMPT line

{
  "transaction_id": "GRC-SINV-42",
  "invoice_number": "GRC-SINV-42",
  "invoice_kind": "B2B",
  "invoice_type_code": "381",
  "issue_date": "2026-07-08",
  "due_date": "2026-08-07",
  "payment_status": "PENDING",
  "customer": {
    "party_name": "Mollo Industries",
    "tin": "12345678-0001",
    "tax_id": "2522500000001",
    "email": "[email protected]",
    "telephone": "+2348012345678",
    "postal_address": {
      "street_name": "12 Marina Road",
      "city_name": "Lagos",
      "postal_zone": "101241",
      "state": "NG-LA",
      "country": "NG"
    }
  },
  "line_items": [
    {
      "description": "Advisory retainer, July",
      "quantity": 1,
      "unit_price": 250000,
      "line_item_type": "SERVICE",
      "isic_code": "7020",
      "service_category": "Management consultancy",
      "vat_type": "STANDARD"
    },
    {
      "description": "Statutory filing fee (pass-through)",
      "quantity": 1,
      "unit_price": 40000,
      "line_item_type": "SERVICE",
      "isic_code": "8411",
      "service_category": "Public administration",
      "vat_type": "EXEMPT"
    }
  ],
  "payment_terms_note": "Payment due within 30 days."
}

Line	vat_type	Net	Rate	VAT
Advisory retainer	STANDARD	250,000.00	7.5%	18,750.00
Statutory filing fee	EXEMPT	40,000.00	0	0.00
payable_amount	—	290,000.00	—	18,750.00

Payable = 290,000 net + 18,750 VAT = 308,750.00. The EXEMPT line contributes 0 VAT regardless of vat_mode — the single VAT rule from §5.

C · INCLUSIVE mode — VAT backed out of a gross price

vat_mode INCLUSIVE · net = charged / 1.075

{
  "transaction_id": "GRC-INC-09",
  "invoice_kind": "B2C",
  "issue_date": "2026-07-08",
  "vat_mode": "INCLUSIVE",
  "line_items": [
    {
      "description": "Diagnostic session (price shown incl. VAT)",
      "quantity": 1,
      "unit_price": 10750,
      "line_item_type": "SERVICE",
      "isic_code": "7020",
      "service_category": "Technical services",
      "vat_type": "STANDARD"
    }
  ]
}

Charged (incl.)	Net	VAT
10,750.00	10,000.00	750.00

Net = 10,750 / 1.075 = 10,000; VAT = 10,750 − 10,000 = 750. Never × 0.925 (rule §11).

D · Credit note (380) — corrects invoice B, abs amounts

type 380 · positive amount · reference points back to GRC-SINV-42

{
  "transaction_id": "GRC-CN-07",
  "invoice_type_code": "380",
  "invoice_kind": "B2B",
  "issue_date": "2026-07-08",
  "reference_invoice_number": "GRC-SINV-42",
  "reference_invoice_date": "2026-07-08",
  "customer": {
    "party_name": "Mollo Industries",
    "tin": "12345678-0001",
    "email": "[email protected]",
    "postal_address": { "street_name": "12 Marina Road", "city_name": "Lagos", "postal_zone": "101241" }
  },
  "line_items": [
    {
      "description": "Advisory retainer adjustment",
      "quantity": 1,
      "unit_price": 50000,
      "line_item_type": "SERVICE",
      "isic_code": "7020",
      "service_category": "Management consultancy",
      "vat_type": "STANDARD"
    }
  ]
}

The reversal is carried by invoice_type_code: "380" plus the reference — the amount stays positive. A negative unit_price is never sent (rule §11).