Family property issue

We are having a bunglow which was buyed by my grandfather in 1965. my grandfather have 6 child among them 1 was girk who died in late 2000 she had four kids. issue is property was on name of my grand father after his death late 1996 one of his elder son visited the society and given his death cerificate and transfer his name as owner. now total leagla partner of house is 9 there is no will our grand father have prepared. when we asked to society committie member about the same he told us that grandfather told them to put his elder son name as owner after his death without any legal procedure. now the elder son who have done fraud he had done some registerd banakhat in 2007 for the same bunglow which we have come to know in 2019 may when we all wanted to sell house. as per him he have just done banakhat not sell we have checked banakht and it was of rs.1451000 and in banakhat we have notice that the person who had done banakhat have issued cheque of 100000 rs to each partner where they even dont know about the same. now as per banakhat it was of rs 1451000 but when we checked bifercation of amount its showing that buyer have only given 100000 rs amount rest 451000 amount was not paid and in continuty there was a agreement where they have mention that if rest amount will not pay within 12 months banakhat or document wiill be invalid. now when we visit the buyer who have our original share certi and all documents is asking for 2cr rs. please suggest what we can do in thease.