How to use Sum with If

[#1]

Hi!

I am new at QV!

When I try to do an chart with following expression I get wrong figure. Could somebody help me, what I done wrong?

sum(Sales)                                                  = 100 Correct

sum(if(BuyingFunction = '1',Sales))             = 120 Not correct, should be 80, because we have other BuyingFunction. How can it be mor than 100?