Unmatch an invoice from a purchase order:
Click Invoice Register, then click Update Invoices.
Find the invoice and click the blue ellipsis button (three dots).
If the invoice has been rejected, use the Amend Entry button to edit it and clear the Order Number field. This enables the Match to Purchase Order option.
Select Match to Purchase Order.
Click Yes to the confirmation message.
Search for the orders.
Clear the Matched checkbox and click Save Match for all relevant orders.
The invoice is now unmatched.