It is dishonest to keep extra change.
Will receives an extra twenty dollars from the cashier while buying something.
Will wants money to buy a meal at a restaurant.
Will gives the money back and borrows money from a friend to go eat.
Will keeps the twenty dollars and goes to the restaurant.
The cashier's drawer is balanced at the end of the day.
The cashier is twenty dollars short that evening and has to pay it from his own wages.