Cancel an invoice

Cancels a partner invoice, with a reason.

reason is required (422 sourcing/invoice_cancel_reason_required).

The invoice number is kept. GST requires a consecutive series, so a cancelled number stays spent and its invoice stays readable as cancelled; reissuing it would put two different documents under one reference, which is what consecutive numbering exists to prevent. A hole in the series and a reused number are both audit findings; keeping the row is neither, and the reason is what explains it to whoever reads the series.

Refused while anything is settled against it (409 sourcing/invoice_already_paid). A cancelled invoice with a payment attached is unresolvable: the money left, and the document saying why no longer stands. The guard reads net settlement (sum(amount_paid), contras included), so reversing the payments clears it; an invoice can be cancelled once nothing is settled against it.

The accruals behind it return to accrued, so a later run can invoice them again. The work was done and the partner is still owed for it. Only the document was withdrawn.