https://www.xperience-group.com/news-item/how-to-allocate-a-credit-note-or-payment-on-account-to-an-invoice-in-sage-50/
If you have a customer or supplier invoice (SI or PI transactions) currently outstanding that need to be paid off by an existing credit note (SC or PC transactions) or payment on account (SA or PA transactions), you can allocate these together.
Bank A/C Ref – Choose the bank account you want the payment to post to.
Date – Enter the date required for the allocation. Normally the later date of the transactions being allocated.
Cheque No. – Leave this box blank, as this does not create an additional transaction.
Payee – From the drop-down list choose the relevant customer or supplier record.
Cheque value – Leave the value as 0.00.
For each transaction you want to allocate, enter the payment details as follows:
NOTE: If the amount allocated to the invoice is greater than the amount allocated to the credit or payment on account, the difference is posted as a payment on account.