The basic steps to issue a refund for an invoice you have taken payment for, is as follows:
Issue a negative invoice (credit note) to the sum of the invoice to be credited.
Reference the invoice of sale to to the credit note.
Mark the invoice for credit as Paid.
Refund the customer using the specified payment method.
Tudodesk makes this relatively simple.
Open the invoice to be refunded. This invoice must be marked as Paid.
Click the 'Issue Refund' button in the top macro bar.
Confirm the details for the refund. You can add the reason for the refund in the additional info box.
Click 'Create Refund'
This will open a "Credit Note" with the following effect
details all the information from the Invoice
Shows reversed negative values
Links/references the original invoice for your record.
Click the Pay button or click Paid in the document status line
Notice the payment amount is a negative refund.
Click Accept Payment.
You will have the option to print or send a digital receipt.
This has now cancelled the original invoice of sale with a negative credit note and refund.