I think its bad karma to benefit from someone else's loss. I would turn the money over to the police in the town where it was found. If it becomes unclaimed property I would donate it to the nearest homeless shelter and place an ad in the paper detailing the amount and how it was dispursed.
I once removed a heafty stuck deposit bag from the night drop of a bank and promptly deposited it back in the box, no one ever knew.
I found a huge roll of cash behind someones glove box while doing a heater core job. Turns out it was 5 grand and the car owner guessed it belonged to the previous owner.
It sounds hokey but I do belive what comes around goes around.
Five years ago I sold two cars for cash and was on the way to the bank to deposit the money. I broke my own rule by stopping at the car wash on the way to the bank. I got out of the car to grab a coffee while there. You can guess when I got to the bank I could not find my bank bag. I was in a complete panic. I called the cops who interviewed all the workers at the car wash gas station, no one saw any bank bag. That evening I get a call from someone asking me to meet them in a parking lot at a nearby shopping center to return the lost bag. Some scruffy looking biker type gets out of a ratty old pickup with paint and ladders all over it. He would not take a reward and just split as soon as he handed me the bag.
I opened the bag in the car and all 16 thousand dollars and all the checks were present.
I can't begin to describe my relief.