Reverse Credit Card function, need formula to calc. current balance i am OWED

yes, I know that, I can just reverse debits to be credits, but I still need a way to have a fixed interest rate being calculated on a changing balance on a monthly basis and then adding that amount to the balance. All the other formulas I have found tell "how many payments till 0 balance, etc" I am not concerned with # of payments, just the remaining balance after each payment.

any ideas?

Check out the NPV function,

Ed