Need Help Creating a Formula

I nee help developing a formula. I have an extremely large database of sales and I need to figure out how to mark the refunds to the matching sale.Here is an example of what my database looks like

Cust # qty price
12345 15 5
12345 20 10
12345 20 -10
12345 20 10
12345 15 10
23666 20 -10

I want to mark down the negative # and ONLY 1 corresponding positive #. The customer number and the qty must match as well. Thanks