Signed a empty stamp paper

Hi all, Seeking your advice badly to come out of a deadly situation. Here's the Case: In a business dealing my father used to sell goods to a person B on regular basis. A person C(friend of my dad) seeks help from my dad to sell his goods and my dad directed C to B. C sold goods for around 20 lakhs and B started cheating without returning the money to C. This bastard C threatened my dad and got signature from my dad on a stamp paper and on a paper written as my dad got 20 lakhs as debt. Later we tried a lot to get money from B where we couldn't succeed. Later we filed police compliant on B saying he has cheated the money and C is threatening with the signed paper. Now, last week, C has sent out a court notice asking for 2,68,000 rs with interest stating that the money has to be repay for getting the goods from C. Please advice how to proceed this case. 1) My dad never took any goods from C. 2) B has given a cheque of 10 lakhs to C and we have that cheque now. Also the bills for the good sold by C to B, we have that now. 3) We have all the copies of police compliant on B. Thanks and Regards, Ezhilarasan Palanivel.