Legal analysis: this is invalid, because you signed it first, and the content was added later, that is, the added content is not necessarily the true meaning of the parties. If it is not true intention, the contract has no legal effect. However, if I can't prove that the content was added later, it's not what I really mean, and I may have to bear the legal consequences of unfavorable proof, that is, the IOU may be considered valid.

Legal basis: Civil Code of People's Republic of China (PRC).

Article 143 A civil juristic act that meets the following conditions is valid: (1) The actor has corresponding capacity for civil conduct; (2) the meaning is true; (3) Do not violate the mandatory provisions of laws and administrative regulations, and do not violate public order and good customs.

Article 490 Where the parties conclude a contract in the form of a contract, the contract is formed when the parties sign, seal or fingerprint it. Before signing, sealing or fingerprinting, one party has fulfilled its main obligations, and the contract is established when the other party accepts it. A contract shall be concluded in written form as stipulated by laws, administrative regulations or agreed by the parties. If the parties do not do this in writing, but one party has fulfilled its main obligations and the other party accepts it, the contract is established.