Filtering in portals problem

I need some help filtering in a portal
I have 2 portals
1st shows the credits and debits for a booking all from table cashbook.
2nd (1 line) shows the totals of the credits and debits.

Portal 1 setup: filter/specify is: If ( Cashbook::currency="$" ; Cashbook::amount_credit; 0 ) and works fine.

I really need to filter out the debits at the same time. (I'll add another portal for the €s later)

cashbook::currency is either €, $ or blank

Portal 2 setup: I have another field: cashbook::total_credits_debits, can you tell me the calculation to add to that field? I tried (cashbook::total_credits - cashbook::total_debits) but that failed.

Level beginner + (maybe)

Henry