How to not net the open invoice balance once Goods return is done against PO

For example, PO line is for 1000 units, and GR is done for 1000, now goods return is posted for 500. At the invoice verification level, i am expecting system to show 1000 and -500 as separate open balances so I can post Invoice and credit memo. But system shows net open balance. How to enable the system to not perform netting for GR balance? Thanks a lot in a advance for your suggestions.