How to subtract a session value from a database table value in a recordset

I have used data assist to create a search able list of my users, and then a way to set up a bill them.

When I bill them a record in the database is made, which has their id in a id field.

Then, on the users cp, I filter that table for the records which has their id.

This way they see there bills.

I then made a PayPal button which is dynamic for the billing amount and so on.

After they pay they come back to my site where I want a simple calculation to be done.

I create a session var which they click the PayPal button for the amount.

On the success page, is there a way to take the session value, and subtract it from the another value in a recordset used to show their overall balance.

Hope I explained this well enough, if you need more info just ask.

Thanks...