Go to Contacts, then open the required contact record.
On the Activity tab, open the allocation transaction created when you linked the invoice and credit note. The transaction date matches the original allocation date.
Clear the checkboxes for the invoice and the credit note you want to unallocate.
Click Save.
EXAMPLE:
If an invoice is $50 and a credit note is $100, the amount to pay shows -$50. You can adjust the credit note to unallocate specific transactions.