let's say there is an invoice which has been paid partially. Is there a way that can show the amount remaining that needs to be paid? from what I've read, this is not available in NS and must be customized individually?

Have you tried using the 'Amount Remaining' field in a saved search on the invoice? If memory serves, that's the way to get that.

yeah I did. worked like a charm. but I just got a new requirement: how do I populate this in the purchase order? I followed SuiteAnswer ID 74638 but it only populates in the saved search and not the field I created in PO.