Unregistered Flat purchase Agreement on 1981 on Rs 5 Stamp Paper.

I purchased a flat in a Coop. Housing Society in Kandivali Mumbai in 1981, the Agreement was drawn on Rs 5 Stamp Paper. The Seller was Fist buyer of the Building constructed in 1976. I have original agreement and also Form VIII Application U/S 27 (2) for permission for Transfer of Urban Land (Ceiling and Registration) Act 1976, duly stamped and signed dated 22.7.81 by Clerk to Additional Collector-Competent Authority (D.J.C.) Greater Bombay. It has IW 9486 as a reference too. I do not know if it is good enough registration or is my Agreement considered UNREGISTERED. I lived in my flat from 1981 to 1990 before moving overseas. The flat is since locked. I am paying society charges regularly. The Building is being taken over by a builder for Redevelopment under a Memorandum of Understanding who also undertook task of Procuring Deemed Conveyance. The builder claims to have paid stamp duty on 2016-17 valuation with penalty suggesting my Agreement is not Registered and the amount will be deducted out of our entitlement. Quite unfair but not sure what is course of rescue