To post a credit note for an invoice already posted through the Purchase Ledger for either the nominal or office follow the below steps:
From The Main Menu select General Postings.
Select PCO or PCN.
Click on NEW.
Select the Matter or Nominal required and also the Supplier this relates to.
Fill in all the details of the credit note and then select the Floppy Disk Icon to Save.
Select POST.
This will make the credit note available to select when next paying the supplier and can be paid with an outstanding invoice.