How to do calculations and arrive at balances

Hello,
It is a calculation that I want to do.
I have repayment data which may be monthly, quarterly, halfyearly say B
I have to pick up the data(B) from one sheet arrive at outstanding balances (ie opening- repayments) . and on the balances another calculation will be done.
To do stimulation or analyse the data I may either increase or decrease the amount of repayment.
In that case , my calculation sheet should not pick up repayments once outstanding becomes nil.