What would you have done?

What if it was larger? Say $40 or $80?

You should have returned it to the teller. By returning it to the manager, you jeapordized the teller's job. You mentioned in the story that the teller tried to stop you. That means that the same situation has happened before and she can't risk another time.

If it were me, I probably wouldn't return it. A bank error to my benefit. I would return it if I had an income, but I'm still a kid, and that's like 3 weeks of lunch money. Dx