1. Was there any loan agreement signed by and between both of you wherein the said high rate of interest was mentioned?
2. If yes, then he said term is illegal since nobody can commercially do lending business with out obtaining RBI license and also can not charge interest at ate more than that has been specified by the RBI.
3. Have you collected the evidence of your repayment? Did you make the repayment through cheque?
4. If yes, then send him a legal letter asking him to to return you the blank and undated security cheque since you have already paid th dues.
5. Post the above letter to your Bank asking them not to honour the said cheque if presented marking it to the local police station also.
6. If he files 138 case, you have enough ground to defend yourself. The main ground is that no cheque bounce case as per Negotiable instrument Act is applicable of security cheque.