Hello,

Concerning the receipt, if you have confirmed it (Marked as Done), then you cannot cancel it. You could delete it, but i highly do not recommend that as it will wreck havoc with your stock and movements tracking. Even if it was just a test receipt.

you can delete by clicking on the Gear icon in the receipt

The proper way would be to create a Return Receipt (you can do that by clicking on the Return button inside your receipt form).