What happens if a customer pays after I've written off the debt?

Two parts to handle. First, record the payment normally — but because the original invoice is voided, you'll need to recognise the recovery through a different account (typically a "Bad debt recovered" income account, or reopen via a new invoice). Second, HMRC requires you to repay any VAT relief you previously claimed under VAT Notice 700/18. Blankitt automatically raises a sibling negative-amount claim on the recover-provision path, so the next return reverses the Box 4 boost you got when you wrote the debt off. This repayment is mandatory — don't disable it.

See: finance-bad-debts-vat-relief, finance-bad-debts-specific