Hi,
After a proper sale, your sale order will have a delivery and an invoice.
- In the delivery, click on “return” and set the quantity as 5, now a new transfer is created back to your warehouse for that product with quantity 5.
- Validate the transfer so those 5 products are back in your stock.
- Now go back to the sale order and select the invoice.
- Inside invoice click on “Add Credit Note”, specify the reason in pop up then click on reverse
- A draft credit note will be created, edit it and change the quantity to 5 and click on post.
- From this point you have two options,
    + You can refund the full amount to the customer by clicking on the register payment in credit note.
    + You can leave the credit note posted without registering payment and use that money as payment for another sale with the same customer.

Regards