Unable to 'Finally Close' the Purchase Order

Hi All

The Purchase Order amount is 5000 Out of which 3000 has been receipted and Invoiced and Paid. Now the User is trying to do the 'Finally Close' on this PO but an error message is showing as:

Error:    You cannot finally close this document because there are outstanding invoices and/or credit memos.

PO User Guide says

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Final Close

Prevent modifications to or actions against completed documents, lines, and shipments by final closing them. Final-closed documents are not accessible in the corresponding entry forms, and you cannot perform the following actions against final-closed entities: receive, transfer, inspect, deliver, correct receipt quantities, invoice, return to supplier, or return to receiving.