Ok I made a test database and I think I resolved this. I am posting my results here to help others if it happens to them. To use a payment as a prepayment, you MUST have a check no, even if you just write "cash-5/5/14" or similar. I don't know why this is, perhaps it is a bug in the software? So just make up a check number and use that, Then it should work.
↧