Keeping something that isn't yours is stealing. The money doesn't belong to you just because she made a mistake. If worst comes to worst and I was you I would probably just run with the story of how you gave it back when she came to your door. However there is the possibility that her boss had her go to your door personally and told her not to collect the money because he needed to see it returned in person so that you couldn't lie about it.
It's a hard decision to decide to keep money or not, but in a case where it's this much money then they aren't just going to let it slide, as opposed to 5 dollars at Burger King.
It's a hard decision to decide to keep money or not, but in a case where it's this much money then they aren't just going to let it slide, as opposed to 5 dollars at Burger King.