I have a similar application, only I substract two currency values.
SELECT Dispatch.Source, Sum(Dispatch.AmtBilled) AS SumOfAmtBilled, Sum(Dispatch.AmtPaid) AS SumOfAmtPaid,SumOfAmtBilled-SumOfAmtPaid AS Expr1
FROM Dispatch
GROUP BY Dispatch.Source;
Dispatch is my table and source, AmtBilled...