This article covers the steps to take to unallocate a receipt from an invoice.
It may be that the user made an error in allocating the receipt to the wrong invoice, or that it is necessary to allocate the receipt to a different invoice.
...
Steps
Go to the relevant Account and click on the Finance Details tab.
You will see the Invoices, Receipts, Credits and Payments on the Account so far.
Click on the “Allocate” button:
...