The gods want me to eat Arby's - and apparently, to pay cash

Even though I packed leftovers for lunch, my gut told me to get Arby's instead.  I went there with a ten dollar bill, ordered a sandwich and a small fry, and received a ten and four ones in change.  Of course, I didn't notice this until I was back at the office.  I am now wrestling with the ethical dilemma of returning the cash or pocketing it, and thanking my good karma.  Unfortunately, something tells me that if I keep it, I'm daring karma to bite me in the ass.  I also don't want anyone to get into trouble -- I picture a stern discussion with the manager, with horrible allegations of stealing nine dollars; the cashier crying into packets of Horsey sauce. 

Honesty will win out yet again, but at least this time it made for a fun little lunchtime story.