In case you need to reimburse a customer and it is not possible (anymore) to cancel the original invoice, you can create a credit note.
Credit notes are invoices with a negative amount and they a very easy way to correct your accounting.
Here is how to create a credit note:
Go to the menu Products > Articles and create a new article.
You can name the article "Credit note" or "Refund" for example.
Enter a negative amount and choose the same bookkeeping account (VAT) as for the initial sale.
Sell the article to the customer that you want to refund and choose the correct payment method (i.e. bank transfer). If you will be paying the customer in cash, you choose the payment method CASH.
You can also edit the amount of the credit note before concluding the sale.
The invoice will then be automatically created.
You can now transfer the amount to be refunded from your bank account to the customer's bank account.