Heavy deposit query

Dear Sir, I want to take 2 flats for heavy deposit to stay without paying rent in MHADA building in Subhash Nagar, Nahur, Mumbai. The owner asked me to pay three and half lakhs as heavy deposit .I want to register the agreement for this heavy deposit. But the agent who introduced me to the owner says that heavy deposit agreements cannot be legally registered. He says that we can just do a notary type agreement. Is such a notary type agreement legally valid especially since i am paying heavy amount. I have to vacate my present house in 3 days and move to another house. so, kindly send me an immediate reply