1. You should have been little careful and insisted for the rental agreemnt executed.
2. When you gave Rs.50,000/- to the owner by cash, was there anybody witnessing it and if so the witness may vouch for your payment of Rs.50,000/- to the owner by cash.
3. Even if the owner refuses to give anything in writing, ask him to give a post dated cheque for the amount due to you. By this, you will have some kind of record for the transaction.
4. Have you already vacated the house. If not, do not vacate till he refunds your entire money.
5. If you have already vacated the house, you can also send a legal notice for recovery of your amount.