credit given for registered user

one of my clients suggested a need for a credit system of sorts -
visitor or registered user would pay a certain amount for a service,
and could pay more than the fee to get credit for future services rendered.
i pay $35 to have a translation done , a one time fee for the translation.
but i have lots of translations to do, not just one.
so if i pay 100.00 i can get four translations done, as an example,
and save on the future services...
has anyone seen a plugin that does this sort of thing?