Send a final demand letter to the landlord this week. Let him know what you're seeking and tell him that you're going to file suit in Small Claims court on X day if the money hasn't been received. Make sure you comply with all of the terms (get him the keys and demand the money when you give him the keys, that is the best bet). If you've already given him the keys, that's fine too. Just make sure he can't say that you were precluding him from leasing the premises.