Subtracting a summed number from a number plugged in from a separate table on a RPT

[csuerun]

This problem sounds so easy, I don't know what I'm doing wrong.

Each dept has a DEPT CODE number
Each DEPT CODE number has several SUBCODE numbers
Each SUBCODE number needs to be totalled (the total is called Expr1 in
the QRY)
Each DEPT CODE number needs to be totalled (the sum of all the
SUBCODES), which is called TOTAL ACCOUNT EXPENDITURES on the RPT
(which is: =Sum([Expr1])
Each DEPT CODE has a REVISED BUDGET which is a number that is plugged
into a table (an Excel file exported into Access) and dumped into the
RPT
Each DEPT CODE number needs to know how much budget they have left
(BUDGET REMAINING):

REVISED BUDGET minus TOTAL ACCOUNT EXPENDITURES = BUDGET REMAINING

I'm thinking Unbound Control, but I can't get the expression right.

Can anyone help me?

I'd appreciate any ideas.

Thanks so much,
Cindi