"Steve" wrote...
Want a cell to enter "0" for values = to 0 or <0, and then take the
positive
difference in that same cell and put it into another cell.
....

X is the balance, Y is the payment, use

=MAX(0,X-Y)

for the balance after applying the payment and

=MAX(0,Y-X)

for the credit balance after applying the payment. Prior to the last
payment, the former will be positive and the latter zero. Just after the
last payment, the former will be zero and the latter either zero or
positive.