Fraudulence against an unregistered document for a value 15 LAKHS

Dear Sir, I made an agreement with a close relative to buy his Property worth Rs.35 Lakhs, and paid Rs.15 Lakhs as advance, and the remaining amount is to be paid off with in an year. We prepare this document on a Stamp paper of Rs.100 which is signed off by both of us with witnesses, but it was not registered, as he was a close relative and I trusted him. The date as mentioned was on due in 40 days from now. Now the said person is refusing to sell his property to me and demanding very high value(Rs.50 Lakhs) for his property, against the terms mentioned in the document. And even If I demand him to return the advance paid he is not refusing too. Every possible attempt to convince him through relatives and friends are failed. He is clearly intended to cheat on me as he is (Wrongly or rightly) confident that unregistered document will do no harm to him. Now, I want to know, what are the legal action I can take against him to either recover the advance paid or to acquire the property as per the agreement terms. Now, I realise that I was a stupid to pay such a huge sum without registering, but I did already, I have to retrieve it back in any way. Who I have to approach for a legal action. The court through a lawer or to register a complaint in Police station or to complain in ECONOMIC OFFENCES WING? Please advice.