How do I sum one type of pmt and with another sum and record but end up subtracting?

[Solly Rosenbaum]

I have 2 methods of payments: check and lockbox. Checks are summing when I select the check method. When I select the lockbox method I want it to sum it and record it, but in the sum I want it to be deducted. So, something like if selected lockbox it will add and keep it recorded and in the end of the month which is a crosstab - to be deducted. Please advice how can it be done. Can you add a condition or a if after a "sum"? what would you advice as a solution?

[gnawoncents]

I'm not entirely clear about what you mean. Please give an example of what you're trying to accomplish.