If you are Posting directly on the Purchase Order, whatever values you see on the lines (totals) will be what is posted on the Invoice. If the costs change for example, you need to update the PO lines to match the Vendor's invoice. This way when you post it will have the correct costing. In order to update the line quantities or cost, the PO must be in Open Status.
If you are getting an invoice for a partial amount or only specific lines of the PO, you can change the "Qty. to Invoice" field to match what is on the Vendor's invoice. The PO can remain in Released Status when making these entries.
Just remember, you're in control. Whatever you enter on the PO lines is going to be what ends up on the Posted Purchase Invoice.
I hope this helps. If so, please mark it as a Verified Answer.
Best Regards,
Ben Baxter
Accent Software Inc