You can go to the Anti-Corruption Bureau of the Procuratorate to obtain relevant evidence (your testimony, his confession, etc.), as long as it can be proven that the reason for the formation of the IOU is hush money, rather than a true creditor-debt relationship. The court would not support his request. If you cannot prove this, it does not matter whether there is a creditor on the IOU. Whoever holds the IOU can claim the rights unless you can prove that he is not the real creditor.