Non possession of flat although full payment done as per the agreement

I have purchased the flat under construction in Goregaon West on resale from a person who had a registered agreement of the said flat in his name. At the time when i purchased the said flat from him it was 95% ready and therefore my bank asked the said seller to pay 100% amount of the agreement and take the No dues certificate and NOC from the builder. Despite of the 100% payment and No dues certificate and NOC the builder has denied the giving possession of the said flat on the premise that I am liable to pay for fungible FSI of balcony which was already there as per the plans attached with the registered agreement.
Further the old society has also denied for taking physical possession of flat on argument for non payment of additional money demanded by the builder.
It is my humble request to suggest me the way out of the same!!