In terms of the mathematics, -13.28 - 13.28 SHOULD result in a value of -26.53, not zero.
-13.28 + 13.28 = 0
13.28 - 13.28 = 0
So if -25.53 is not the expected value, you need to figure out if the error is that the first term should not be negative or if you should not be subtracting the second term.
99% of the values are positive... so Round ( (IN_amount - INPaidAmount);2); works.
in the 1% that are negative it does not.
What can i stick in the calculation that will accommodate the 1%?
Round ( Abs ( in_amount ) - inpaidamount ; 2 )
My point was that it DOES work. It's not producing the value you expect it to, but it is producing the amount a mathematics text book will tell you is correct.