Tenant fraudulently seeking possession over flat after fathers death

Sir my father built a house by taking employees personal loan and we gave the house for rent and for financial difficulties we taken small amounts from our tenants the amount now is about 130000 rs and my father and the tenant came to an oral agreement that no interest for debt and no rent for the house.my father expired in 2013 and we went to tenant and said that we will give the money and you vacate the house.then he showed us a 100rs stamp paper stating that agreement of sale and rights over the house.it is stating that 328000 has paid to my father and total amount is paid.he forgered my mother sign after knowing my fathers death.he corruptly changed the muncipality and current under his name.so what do we do now .he submitted a case stating me and my mother he filed my name wrong in the case and in the agreement he wrote my grandfather name wrong.so who will win in this case .and please tell me our positive and negative points in this case.i would be very happy to hear reply fast.