Registration of flat

Sir, I hav bought a flat in mumbai in 2007 in wich we was on rent for 6 years on a single agreement.later whn the owner planned to sell the property, we agreed to purchase the same for which i hav paid 80% of the deal amount to the owner.He promised to tranfer ownership into my name within a year after which i hav to pay him remaining 20%.He presented a power of attorney in which he clearly mentioned the above said deal.After a year he left conatact with us n later whn we tried to contact him he was not ready to come n finish the deal.Nw its being 9 years he is still nt ready to cum as he wants the current market rate of the property.This apartment is 27 yrs old n till the date no tax was paid later whn the muncipality left a notice we paid the tax along wid the fine n nw the tax bill cums in my name after dat i tranfered electricity bill into my name. Nw the building is going into redevelopmnt and the flat is nt yet registered 2 my name. can u give any solution 4 the above said issue.