If the other party's claim is invalid, unless it can be proved that the IOU was written under coercion and other circumstances that could not express the true meaning of the writer, even so, it cannot be said that it has no legal effect. In contract law, this IOU can be revoked, and the revocation must be decided by the court. Of course, it is invalid ab initio after revocation.
If the repayment period is clearly stipulated in the IOU, the IOU shall be valid for 2 years from the date of repayment period. If there is no clear repayment period, it will be valid for 2 years from the date when you first claim your rights.
If you bring a lawsuit to the people's court, there is no problem in winning the case. In practice, even if the case is won, the money may not be returned, and the other party may maliciously transfer its own property, resulting in the judgment being unenforceable. I suggest you find out his property status first, and apply to the court for litigation property preservation when suing, so that he can't use the property for disposal and protect your legitimate rights and interests. In addition, to apply for litigation property preservation, a certain amount must be paid to the court as a guarantee. If the other party's property is wrongly preserved (that is, you lose the case), the deposit will be used as compensation for the wrong freezing of the other party's property.
Legal basis: Article 470 of the Civil Code stipulates the contents of the contract, which generally includes the following clauses:
(1) The name and domicile of the party concerned;
(2) Subject matter;
(3) quantity;
(4) quality;
(5) Price or remuneration;
(6) Time limit, place and method of performance;
(7) Liability for breach of contract;
(8) Methods for resolving disputes.
The parties may conclude a contract by referring to the model texts of various contracts.