You can add a credit note to a supplier

1. Search for the supplier
   
   
2. Scroll to the credit note section and click Add new credit note
   
   
3. Fill in the form as required and click Add credit note

You will need to allocate this credit note to a supplier invoice.

1. Click View on the credit note
   
   
2. You will see a section for allocation history. Click Add new allocation
   
   
3. Change the information at the top of the screen as required
   
   
4. Select the invoice from the drop-down, or in the find it in the list (it will be in the list already if it is overdue)
   
   
5. Type the amount in the 'amount to allocate' column
   
   
6. Remove any unnecessary rows using the Delete button. Click Add cash allocation

Please note, if the invoice was assigned to a job, the credit note will assign to that job also