Class on Ethics

A father is explaining ethics to his son, who is about to go into business.

"Suppose a woman comes in and orders a hundred dollars worth of material. You wrap it up, and you give it to her. She pays you with a $100 bill. But as she goes out the door you realize she’s given you two $100 bills."

Now, here’s where the ethics come in:

should you or should you not tell your partner?