Pro-forma invoice
(Pro-Forma-Rechnung)
In some restaurants when you ask for the bill, they will bring you a bill with the words "pro forma" printed on it. This is not a real bill, but just a mock-up of the bill, which has no legal standing.
So why do restaurants do it?
If they printed a real bill, they would owe the VAT on it and would have to tax the net amount printed on it, even if you refused to pay it.
So they are being careful, and will only give you a "real" bill, i.e. one without the words "pro forma" on it, after you have paid.