If you mean can you or your customer cancel a transaction (sale) after it's been completed, the answer is no. You can do a refund, but you can't cancel (or delete) the sale.
If you mean can a charge or debit be canceled, the customer can enter a dispute against the charge/debit in which case the user's card-issuing company will investigate, and Square will help you out as much as possible to get it resolved in your favor.
