What is the use of a bank statement when the person is saying that he has paid in cash?
Bank statement would not prove anything except that you paid him money by cheque which even your bank statement can prove. In any case you cannot ask for anyone's bank statement under RTI as it is not a public matter.
When transactions occur without relevant documentation i.e. oral contracts like this, the matter can be resolved in Court by producing evidence. You have evidence that you gave him some money by cheque as a loan and he is stating that he repaid it by cash. You have a strong case since he may fail to produce witnesses. He would also have to disclose how he got the cash. Did he withdraw from a bank account or did he borrow from someone, etc ?
S Jadhav