Thanks @BettyJaneB for your clarification 

BTW I just found that when you run the 'Purchase by Vendor Detail' report you can add a column for 'open balance' which is the 'unpaid' balance of the billed portion from the PO........ (I hope i'm correct)

But it's still not the best option for a construction company since you won't be able to Progress bill only with changing the QTY which isn't always possible (sometimes you can't get to the exact amount you want)

All the best