I would take it back. The money would not belong to me, and it would bother me to keep it. Whether it comes from change from a cashier or a MAC machine, someone is responsible for controlling that money, and it would be an issue to have it missing. Just yesterday, I found $5 in the parking lot at the SPCA. I took it back inside and turned it in. It wasn't mine, and I wasn't comfortable keeping it. Hopefully, the owner came back for it, but if not, I hope they used it for the pets. In either case, I didn't feel right keeping even a lost $5. If I found the money blowing around somewhere where there was no way I could try to find someone to turn it into I would donate it.