Chacha took property of grandfather and now asking money to give it

My grand parents had 3.5 cent. They expired now. Totally 7 children (3 girls, 4 boys). My father is the second eldest. My chacha kept the patta for 3 lakhs (for my grand parents' medical expenses) without anyone's consent. Now it has become 5 lakhs. My father is planning to buy the land. But it seems like more problems are there as my chacha is a fraud (who is already doing real estate business and has cheated many people). Now the total land cost is 14 lakhs. Chacha tells us that my father has to pay 5 lakh in advance to get back the patta back followed by handing over the remaining amount to my father's 6 siblings. But I am really afraid that after giving 5 lakh my chacha will cheat us. Kindly help me how can i sort out this so that my father can buy the land without getting cheated by chacha.