I invoice customers monthly for services, because these services are supplied both ways my invoice is:
(service provided by me less service provided by them) - sometimes I owe them money
The way credits work don't suit me because they are immediately shown as paid. If they worked the same as invoices - ie not marked as paid until they are actually paid in the banking that would suit me
I tried also making them a supplier so I could raise order to them but that doesn't give them any paperwork of how much I owe them.