A person cheated my Father in Land registration

Hello Sir/Madam Since its a big story I will make sure to brief it as much as possible with main points My father was cheated by his own friend on land registration. What happened was my Father's friend started to experience the financial crisis at a point of time. So he approached my father and asked to buy his property from him so that it will help him to come out the crisis which he was facing. My dad accepted to buy the property, my Father's friend informed my father that he has pledged the property document in the bank. So my Father said him that he will pay 85% now and will pay the 15% at the time of registration he also agreed upon the same. my Father paid him as above mentioned with the cash, My father's friend received it and started dragging the process of registration. Later on he(father's friend) filed a complaint stating that my Father threatened him for registering the property and filed a case in the court as well the sad part is that my dad didn't created any document which can say that he paid money for the property case is going on from 2008 and we don't know what to do in this whatever i have said above is nothing but the truth but the court needs the papers as the proof which dont have we are living at the property since we paid him the money. I am not here in the fear of, what if the property goes off from our hand. I am here because that money was hard earned money by my Father If any of you give us a suggestion to finish the problem i will be very grateful to you thank you