Accounting automation that refuses to guess
A fail-closed n8n system reconciles daily SumUp activity before it creates or verifies the matching Lexware revenue documents.
The accounting constraint
Payment data and accounting records were disconnected. A fast automation that posts the wrong document is worse than manual work, so the system had to prove a day before it could write it.
One business day at a time
A month-bound driver sends exactly one business date to an inventory workflow. Inventory combines a control source, SumUp evidence, and a Lexware same-day scan. It persists the day state, transaction records, an immutable batch manifest, and a run log before a worker is allowed to post.
The write boundary
The day worker processes cash and card legs serially. For each one it looks up an exact planned number, checks for same-day collisions, recovers any stored document ID, writes only when needed, and reads the result back to verify it.
Online invoices and credit notes follow a separate exception worker. Each document carries a unique marker so a stopped run can resume without a blind repost.
Fail closed by design
Missing evidence, a payout mismatch, unexpected source data, duplicate candidates, and uncertain POST outcomes block the day for review. Terminal days are safe no-ops on re-run. The goal is not maximum throughput. The goal is correct, traceable accounting.
Reusable lesson
For financial integrations, use persisted state, immutable plans, idempotency markers, collision checks, and read-back verification. They make an automation operable when an API rate limit or a mid-run failure interrupts the happy path.
