Stamp Duty

My tenanted property in South Mumbai was redeveloped and handed over as ownership property. Now after one year of receiving the allotment letter I want to sell it. The building has received OC. The purchaser wants to file the stamp-duty challan online before the 31st March deadline to take benefit of prevailing 3% stamp-duty rates. He is willing to pay me only 10% deposit on the property's ready reckoner rate and convincing me that this is no risk to me as he plans to register the property only after 3mths in early July when he pays me the balance 90% payment. I have the following questions pertaining to this. 1. Is there any risk to me as an owner to allow him to go ahead filing online stamp-duty challan while he holds up money till registration? 2. At the time of registration which will be after 3 mths in July will the stamp-duty rate be applied at 5% or will he get the benefit of the 3% since he filed the challan before 31st March? 3. In the worst-case scenario if for any reason the deal does not go through; do I as a seller have to cancel the stamp-duty paid? If Not then will I be able to sell the flat to another purchaser without any complication since the property would have had stamp-duty already paid once by the previous purchaser? Will appreciate an early reply soon as the purchaser is pressurising us to give him all documents.