Supplier credit notes
1. If the credit note relates to an invoice you have not yet paid:
Make sure the invoice has been created in Supplier Charges. If it hasn't then enter it.
In Supplier charges click the Credits/debits button
Enter the credit amount for the correct invoice.
When you enter the payment for the invoice the amount to pay will be the total amount minus the credit note.
2. If you have already paid the invoice. This is called an "out of cycle" note.
Wait for the next supplier invoice, it should include the credit note's amount.
The follow the instructions for 1. above.