It's a 3 party app that only allows you to credit the bill amount.

So say you bill 4X £100 separately, when you raise a credit you have to cherry pick which bill amount you want to credit. So you can't bill 4X £100 and credit 1X £200 or 1X £500 for example. If you wanted to credit the whole bill, you'd have to credit the 4X £100 individually.