Can you withhold an employees paycheck if he let a customer leave without paying his bill in full?

Not in most states. It is against labor laws & probably same in all states. Call local Labor Relations Board for legal info. Sounds like you run a place where it may be difficult for general employees to police customers.Owner or manager should do that.Suggest employees tell you or your manager when that problem comes up & you or your person in authority does that.have a bell or buzzer near register to alert you or person in charge.Prepare to get police if was done on purpose & not an overlook which happens.