I would NOT keep the money. Integrity aside, just for business reasons. This has happened to me several times. It will impress your client with your honesty when you return it. It gives you a chance to ask your client what it would take to get him back. Find out why he left. He must have been unhappy about something. Find out what it was and fix it.
People want to work with someone they can trust. He will tell others about this "honest" yard monkey and you will get other clients. Every time? No. But I have got referrals from clients I don't even have anymore.
Don't let people say you tried to return it and it is now yours. You did not do the work and the money is not yours. If he told you to keep it it is a different story.
Do the right thing and you will never go wrong!