Formal procedure for the surrender of tenancy rights in Chembur (Mumbai)

I am landlord of a property in Chembur area in Mumbai city, which is occupied by tenants. Below is the scenario for which I am looking for an advice: Tenant left the property in 1984. At the time of leaving, he provided in writing on a simple lined white paper stating that he wishes to surrender the tenancy to landlord for a consideration. He signed the paper affixing a revenue stamp. It was neither a stamp paper, nor the document was registered. We (landlord) paid the consideration. However, we don't have any formal proof of payment as the transaction is quite old (more then 35 years back). Tenant no longer resides in the said property, keys were surrendered to landlord, no one has paid rent for last 35 years. Landlord pays the water and electricity bill. The room was metered for electricity with the connection still in tenant's name, unfortunately it has not been changed to landlord's name. Does tenant still hold any tenancy right on such property? Can landlord sell such property without any encumbrance from the tenant? Is there any requirement to get that old surrendered deed registered on a stamp paper before the sale by landlord?