Three-Way PO/Receipt/Invoice Match and Auto-Approval

A vendor invoice arrives (email, EDI, or portal upload);
the AP system extracts header and line data via OCR or EDI parsing and links the invoice to its source PO by PO number. The matching engine compares invoice quantity and unit price against the PO and the warehouse goods receipt note (GRN) line by line. If all lines fall within configured tolerance thresholds (e.g., ±2 % price, ±0 units), the invoice is automatically approved and queued for the next payment run. Exceptions are routed to the AP clerk for manual resolution. The approved invoice posts a debit to the AP accrual reversal or expense account and a credit to Accounts Payable, closing the GRN liability.

invoices { id: string, vendor_id: string, po_id: string, status: enum(PENDING_MATCH|MATCHED|EXCEPTION|APPROVED|PAID|VOID), total_amount_minor: int64, currency_code: char(3), external_id: string };
invoice_lines { invoice_id, po_line_id, grn_line_id, qty: decimal, unit_price_minor: int64, match_result: enum(MATCHED|PRICE_VARIANCE|QTY_VARIANCE) };
match_tolerances { vendor_class, price_variance_pct, price_variance_amount_minor, qty_variance_pct };
(reference, product may differ).

POST /v1/invoices { vendor_id, po_id, lines: [{po_line_id, grn_line_id, qty, unit_price_minor, currency_code}], external_id } -> 201 { id, status, match_result };
GET /v1/invoices/{id};
GET /v1/invoices/{id}/match-result;
emits ap.invoice.matched or ap.invoice.exception event;
idempotent via external_id.

PENDING_MATCH -> MATCHED -> APPROVED -> SCHEDULED -> PAID;
branch PENDING_MATCH -> EXCEPTION -> APPROVED (after resolution);
terminal VOID;
guard: PAID invoice cannot transition to VOID without void-and-reissue workflow.