As long as customer invoices have not been posted (completed), they can be deleted. However, as soon as these have been completed, it is only possible to cancel.
Procedure #
Switch to the Customer invoices menu(Accounting → Accounts receivable → Customer invoices) and search for the desired invoice. If the invoice has already been closed, you must deactivate the “Only display open documents” checkbox.
You can either cancel from the list: click on Cancel in the three-bar menu for the affected invoice …
… or you can open the invoice by double-clicking on the line or by clicking on Edit in the three-bar menu …
… and then click on “Delete” on the screen. The system decides independently whether to delete or cancel.
Difference “Cancellation” vs. “Deletion” #
The difference is that the document is deleted when you delete it. The invoice number is not assigned again. Instead, it gets “lost”.
When canceling, the system creates a cancellation document. This balances the bill. The cancellation document contains exactly the same amounts, only reversed.