Hi Jerry,
Though i know it's very late to response your query, I would like to add my piece of suggestion
In simple means if you are using on Premise & Supplier Master data comes from ERP
Case 1: If all Invoice to that Supplier is linked to PO
then by using Inspector you need set Supplier & SupplierLocation object Active =False & Set AdapterSource= ManuallyMaintained till all Outstanding Payment against this Supplier is being settled.
Case 2: If all Invoice to that Supplier is not linked to PO
set only the AdapterSource =ManuallyMaintained. and add validation to compare AdapterSource.