Can a landlord change his mind on a lease agreement if he has not signed the lease but got the security deposit .

You probably can, but that doesn't mean they won't sue you. Depending on the length of the lease and what oral represntations were made, they may claim breach of an oral contract and/or misrepresentation and justifiable reliance on their part resulting in some financial harm to them. Given the limited information you provided, its difficault to say whether or not they would prevail with such a claim. The best thing to do is see if you can work out some arrangement with the people you were going to rent to ie higher rent/earlier move in date.