use this dimension: =if(Sales>0,Partener)
It works as designed. If restriction condition is set to "Show only values that accumulate to xxx relative to the total", negative values are not included in the calculation of the total ("Restriction sum", that is the total that is used for evaluating the restriction for a particular row). The reason for this is the following:
Imagine the following case:
The sum for this field is 0, which is undefined when you do relative calculation. What is the 40% limit of the sum 0 in this case? And how many values should be displayed?
The way to handle these questions, that makes most sense is to discard the negative values before performing the restrictions. Doing so, The sum is 3 and the 40% limit is more intuitive (1.2). Thus the value 1 will be shown.
A good is advice is to avoid relative restrictions when there are negative values as this can lead to a behaviour that is difficult to forsee (even though it is well defined)