Voucher is interesting, I hadn't thought of it. But I'm not sure.

Here are the ideas I’ve com up with so far. I thought someone might be able to improve or fix one, but I’m also wide open to not using any of them.

finite credit balances
finite usable funds
finite credit funds
available credit balances
credit holdings
finite principal resources
usable credit balances

None seems descriptive or natural sounding enough, and I need the word or term to be intuitively understandable and as unambiguous as possible.