Is there any way for Excel 2008 to automatic subtract an amount from another
number each month?

Example

A1 has a monthly payment of $500
A2 has the whole cost of $5,000

Is there a way for A2 to automaticly deduct the amount in A1 from the total
amount in A2 and replace it with the new amount of $4,500. Then do the same
each month automaticly?

Trying to work on a Family Budget with Excel and I wanted to be able to have
an estimate of the reminder of each Fixed Bill.

Any ideas?