My first thought was to to say give her half or give her a nice gift, but as I thought it about more, I think that I would end up just taking back the $20 and letting her keep the rest, merely because when I gave the twenty I would have never expected to see it again, so i wouldn't be losing anything -- In fact I'd be gaining $20 and if I was this women's employer I would certainly want to reward that kind of honesty.