How to compute for debit that has negative value without including the credit value?

Hi,

In the vendTrans table, both debit and credit are stored in amountCur.

Example:

Debit: -10 and -20, which means that it's on the left side.

Credit: 50 (this is on the right side)

When I add all the debit, it also includes the credit: 50, which gives the total -80.

How should I compute the debit (-30) only?

Thanks!!