Problem with sale deed

In 1985, housing development board in my home town sold the plots from government owned land. Many applicants applied for it. My father and my uncle were among them. Since, there were numerous applications people were selected in a lot. My uncles application got selected and he was allotted a plot. Then, my uncle decided to give that plot to my father. Agreement was verbally made and my father started paying the monthly installments in my uncles name. After paying the whole amount agreement of sale was signed between my uncle and my mother in 1995 and it was registered. Sale deed was not executed because it was my uncle, and they dint feel the urge to do so at that time. Then patta was issued in my mothers name. We have built a house. we are living there still now. All the water tax, electricity bill other taxes are being paid in my mothers name. Now, recently, my uncle is asking a huge amount to be paid to sign sale deed. Till now, we haven't got any reason to sell our house. He is not threatening. He says that we won't be able to sell our house. And he needs money to sign sale deed. what are the possible outcomes of this case?. What should we do if we were to sell our house?.