(50.00) - (40.00) = (10.00) , A5-B5
50.00 - 40.00 = 10.00 , A6-B6
(50.00) - 40.00 = (90.00) , A7-B7
50.00 - (40.00) = 90.00 , A8-B8

If this does not help please explain your problem little bit more, or attach the sheet in which you are having the problem.

Regards