paidSum defined as
(yourfield = "Paid"; "";
You need to nest the sum function into another function. The CASE function should get you the result you want. Try the following in the field where you want the result to show: Case(PaidStatusFieldName="Not Paid";Sum(SumfieldName)).
In this case substitute the name of the field that contains the text "Not Paid" for PaidStatusFieldName, and the name field you want to sum for SumFieldName.