A common problem with credit notes is that people can think of a credit note as a negative amount of money, but if you try to enter a transaction with a negative value you will find out that is not possible.
A credit note is actually a positive amount of money, so a sales credit note is an amount of money leaving the company, and a purchase credit note is an amount of money entering the company.
Issuing A Credit Note To A Customer
When you give a credit note to a customer, you are paying money out so you treat it as a purchase (money paid out of company).
For example if you have issued a credit note for £100, go to the "Purchases & Money Paid Out" page and enter a transaction for £100.
Receiving A Credit Note From A Supplier
When you receive a credit note from a supplier, you are receiving money into the business, so you treat it as a sales (money paid into the company).
For example if you receive a credit note for £100, go to the "Sales & Money Paid In" page and enter a transaction there for £100.