Thank you! Your answers put me in the right direction of thinking. I don't know how to apply your suggestions but I do understand what you are saying. I'm still learning. For those who are still ignorant of SQL programming you can create the following expression to get it to show the true value. In our case we have PYMNTs that need to show as credits.
=IIF(Fields!Document_Number.Value Like "PYMNT*", Sum(Fields!Current_Trx_Amount.Value)*-1 , Sum(Fields!Current_Trx_Amount.Value))