Excel greater or less than

Hi all,

I'm having a problem that with multi counting with less than, eg.

case 1:
A1 = 1000 (total revenue)
B1 = 3% (rebate)
C1 = 2000 (free copy)

case 2:
A1 = 8000 (total revenue)
B1 = 3% (rebate)
C1 = 2000 (free copy)

I need a formula that when A1 - B1 the net revenue less than C1 the result become 0, but case 2 when A1 - B1 the revenue greater than C1 eg. A1 - B1 = 7760, then 7760 - C1

I wrote a formula like below but failure to work...
=SUM((A1-(A1*B1)), "<=" &C1,0,D1-C1) ---- (get the net revenue)

thank you!!