If the entirety of the sale is correct with the exception of the customer which was selected, you can change the customer by using a Supervisor option on the receipt preview.
- Go to Customer > Find, lookup the customer the sale was incorrectly entered under.
- Select the customer record and visit the ACTIVITY tab.
- Select the sale which needs to be moved to a different customer and select "See Sale"
- When the preview is up, select the Supervisor button on the bottom, then Change Customer.
- You can then select the correct customer this sale should be under.