Broker and owner creating problem in society transfer, please advise

I had purchased a Flat in 2012 in Nalasopara W. I had paid all the money to the Previous Owner of the Flat. We had purchased this flat with the help of Home Loan, We wanted to shift asap due to some family problems, and the Previous owner told us to shift after we made the initial payments, The Home loan took a little more time than it usually takes, but it got sanctioned. The Previous Owner did came for registration and completed that procedure, but the Brokers who were involved did not took signatures on the Society Transfer. When we signed the documents and sent it to the previous owner for signatures, He demanded extra money, He is holding us for Ransom statinh the Home loan took more time but there was no signed agreement between us to pay him extra money if the Loan took extra time to be sanctioned. Society is also not helping us, They are asking us to get the signed transfer papers. Please guide us what can we do about it. Its been almost 5 years now but we have not got justice.