Query on Resale House Agreement on ownership of March 1985

Planning to go for redevelopment in Mumbai. I am presently the flat owner however society was not formed and there is a landlord in our building. The following are the details of the past agreement The flat was purchased in July 1985 from the previous owner of the flat (resale). The agreement was made on INR5 stamp paper. Agreement was not registered then as stamp duty was not there and started effective Dec'1985. Now the developer wants me to pay stamp duty as per current market rate for agreement dated July'1985. Please note that I dont have any residence proofs with me of 1985 as all was washed out in 26July2005 flood in Mumbai as I am living on ground floor except for the agreement. Could you please let me know if agreements as old as 1985 required compulsory registration ? Am I required to pay stamp duty as agreement is of July 1985 If I have to pay stamp duty , what are the ways that I could avoid it or pay minimal (is there a way I can prepare an affidavit for residence proof or something else to prove that i resided in that flat eff 1985 to save on the stamp duty) Please advise a way out , as except for flay ownership agreement , I dont have any proofs before 2005 as all is washed out.