How to issue a CREDIT NOTE
1. Go to accounts receivable - all free text invoices

2. right mouse click on invoice number - view details:

3. select your record - reverse transaction (button might be under arrows)

4. select corresponding lines - ok

5. Inbound/outbound booking – select record – create invoice proposal – invoice proposal
UNAPPROVE – select line for credit note – approve (already invoiced line will stay selected, has no impact)

6. create invoice – view draft
if correct, confirm with ok

7. Now use remaining line to invoice correct amount or suppress line if not needed

If you would like to have more information, please check out the following links:
Invoicing & contracts fundamentals
Invoicing & contracts advanced