...
Navigation: Purchases - Purchase Orders - Review PO Information - Activity Summary
1. Enter "PO number" as appropriate.
...
4. To see all columns you will need to click on an Arrow Tab "Show all columns" (under "Customize" tab) The following information will be displayed on the screen:
Amount Ordered, Amount Received, Open Amount, Quantity Invoiced, and Amt Matched.
5. Open Amount shows a PO balance