Add Sum on column based on customer_id which cannot be used as a Group in Tablix

Hello,

for a product family there are N customers who would have bought the product, There could be negative transaction as well. so for a customer who has 2 trnasction one is positive "10" and other is negative "(-20)" and if i sum them it would be -10, so i just want to count the distinct customer who has positive transaction after sum.

As my report tablix is at Product family level i cannot add customer  as it very detail data, so i would like to have  acondition to check the sum of Amount at customer and check if it is greater then 0 and if it greater then 0 then include the customer in the count.