The books now say what actually happened

Mark paid set an invoice to paid. It did not ask for an amount, did not record a payment and did not touch the balance, so the invoice read as settled while the money was still owed. The same press on Reverse produced a reversed invoice showing its full balance due, which is how this was reported.

Paid, part paid, overdue, partly credited, reversed, written off: none of these is a decision. Each is what is true once something has been recorded, and each already has an action that records it. The server refuses them by hand now, with a message naming the right one, and they are gone from the buttons so nobody has to be refused.

A document that is settled or withdrawn stops offering what cannot be done to it: no payment, no advance, no write-off, no revision on one that has been reversed. Delete appears only on drafts, which is the only thing that can be deleted.

It could only ever be spent on the invoice it was raised against, and the invoice’s credited figure was written by hand from the credit note’s total. Neither survives what businesses actually do with a credit.

Apply it to any open invoice of that customer, for any amount up to the smaller of what the credit has left and what the invoice owes. Take an application back and the invoice owes it again. Or refund it, recorded as money out so it appears on the bank statement. Every application is a row, and the two balances are read from those rows rather than kept beside them.