How do you prepay for an item on a Purchase Order?

I have Document IDs 626688.1, 626921.1, & 627107.1 and still do not understand how to help my A/P Department prepay for a product from an invoice and then upon receipt voucher that same invoice against the receipt. I had set up the PrePayment Vouchering in A/P previously as described in Document ID 627107.1 and it worked fine but it did not involve Inventory and a Purchase Order. So they used the PrePayment Voucher to voucher a PrePayment of an Inventory Item and now the system won't accept the Invoice Number in the Voucher Match program (P4314) as it was used