A client wants to return more items but you can't void the previous Credit Note because the items restocked have already been used.

1. Go to:

 Sale > New Sale (Services Only) 

Create a new invoice with the amount you wish to credit and 100% discount so you end up with 0 due.

2. Create a Credit note for this invoice. This time you will use 0% discount.

3. To restock the item use stock adjustments.