How can I complete a change order for Partially Matched receipts?

We have Purchase Orders that are linked to a Procurement Contract. Receipts were made against the purchase order, but the receipts were partially voucher/partially matched. Departments are trying to close this PO and return the un-invoiced amount to the contract. The only way to release amount back to the contract is by creating a change order on the PO. However, we cannot reduce the PO amount because it has been partially received/matched.  I closed the receipts, but the system still validate against he partially matched status.

Another scenario: There were no receipt created against PO. A PO voucher was created against the PO. The voucher invoice less than the PO amount. This PO line is partially matched. How do I reduce the PO line amount to the invoiced amount to release back to the contract?