The cashier failing to scan in the tip is the same as a manager) losing $20 from the tip jar: because the customer provided the tip to the employer, but the employer then "lost" it, the employer is liable to make it up to the tipped employee(s). When employers cost their employees money or lose employee money, they have to make the loss good, the same way that if you lost $20 belonging to your friend, you'd owe him or her the money. That's the law. Practically, there's nothing to do about it: if the employer will not good on the $20, the only way to get it from them would be to sue them for the $20, which is hardly worthwhile.