I would return the money but keep the bag.
No, I would contact the person and return it. If there wasn't any contact info on the bank bag (they usually have deposit slips though), then I would let the doctor's office know I found something of value, and leave my contact info. I could never live with myself if I kept it.
Now if a bundle of cash or drugs washes up on Miami beach...that shit is finders keepers. Trust me, it happens A LOT in Miami, too.