Only IOUs with an official seal and no signature of a legal person are valid. According to the laws of our country, an IOU that is only stamped with an official seal but does not have the signature of the legal representative is generally considered valid. Unless the unit's defense is that it only issued an IOU, but the counterparty did not fulfill the loan or deliver the goods, and the counterparty cannot provide evidence that it has been fulfilled to the unit, the IOU can be deemed invalid.