Credit notes
An invoice that has gone out cannot be edited, so the way to undo one is a credit note — a second document that credits the first, with both standing on the record. Press Credit on an invoiced request, say how much and why.
A reason is required, because a credit note without one is a hole in the accounts somebody has to reconstruct from memory. You cannot credit more than the invoice was for, and you can credit the same invoice more than once — £100 off in March and another £50 in April is perfectly ordinary.
What it does to the money is the important bit. A credit note reduces what is owed without changing what was paid or what was agreed. It is not a refund, so it never appears in your payments; and it does not rewrite the agreed total, so you keep the fact that £660 was agreed and £110 later credited. What the group owes is the agreed total, less credits, less what has arrived — and the booking says “after £110.00 credited” so the lower figure is never a mystery.
Credit notes are numbered in their own sequence, separate from invoices: sharing one counter would interleave the two and neither run would be unbroken. Like invoices, an issued credit note cannot be edited or deleted, and it can be printed or emailed. It carries the address and VAT position of the invoice it credits, not today’s — so a campsite that has since moved still credits under the letterhead that issued the original.