Invoice

Because a promise to pay needs to be recorded in a form both parties and a court would recognise, at a specific moment, for a specific amount.

The invoice is where a commercial relationship becomes a number in the accounts. Everything before it is intention; everything after it is collection. That is why the rules around it are stricter than anywhere else in the system — and why "just edit it" is the most expensive shortcut available in an ERP.

• An invoice must post to the general ledger; an unposted invoice is a draft, not an invoice.
• An invoice's total must equal the sum of its lines plus taxes, always — no rounding plug.
• A posted invoice is immutable. Corrections happen via credit note, never by editing.
• In India, a tax invoice must carry a GSTIN, a place of supply and an unbroken serial number.

The recurring failure is treating the invoice as a document rather than a ledger event. Teams edit it, delete it, or recreate it, and then cannot explain why the receivables ledger disagrees with the bank. Under Indian e-invoicing the same habit becomes a compliance problem, because the IRP already has the original.

• Partial delivery: invoicing 100 units when 80 shipped creates a dispute the system should have prevented.
• Advance received before invoice — the payment exists with nothing to settle, and must sit against the customer, not an invoice.
• Credit note issued in a later tax period than the invoice it reverses, which changes which return each lands in.
• Foreign-currency invoices, where the receivable revalues but the invoice does not.