Credit Management Module

[Serge]

Guys,
         I was wondering if anyone has worked out a way to process "part payments".

I am thinking of allowing users to buy "credits" and then draw against that credit when they buy at a later date. 

Ideally, as the customer adds items to the cart, the available credit would be decreased showing the customer how much they have left.

Any ideas?
Cheers
Serge