I said above, initially, my first reaction would have been to give her $220 and keep $500. And that honestly was my "first reaction"

However, unless I ran out and bought me a chip ...
as the "boss" - ESPECIALLY after bragging to everyone in the office about "our good fortune", I'd probably take my $500 and either add it to the Christmas Party fund, buy the staff lunch, or if there weren't "too many employees", split the "profit" between my employees so everyone reaped some small benefit from her honesty and a stroke of good luck.