subtraction calculation in RTF template

I am working on an Oracle XML Publisher procurement report (Oracle E-Business Suite R12) that starts with a budget amount that is imported from our financial system, then lists all requisitions and orders that are in process (not approved).  The amounts of all those unapproved requisitions/orders are totaled using a Sum form field, to be followed by a field where the unapproved requisition/order total is subtracted from the budget amount (amount available).  This is the code I used in my form field for the amount available:

<?xdofx:BUDGETAVAILABLE-(sum (EXTENDED_COST))?>

When the report is run, all the fields are successfully populated, except for the Amount Available -- it lists the original budget amount, but does not subtract the sum of the extended cost.