1, don't lie, take the initiative;
2. The IOU is indeed the best evidence, but I don't know if there is interest written on the IOU;
3. The relevant regulations of the People's Bank of China do not allow usury, but stipulate the lending behavior between individuals. Suggest that your lawyer find relevant regulations;
I don't know how long you charge interest, which is directly related to whether usury is established.
5. Consider writing an article in the name of interest collection, for example, it can be broken down into: total interest collection fee = interest+management fee+other expenses, etc. In order to show that he has never received usury.