No one will admit that the IOU was written by the client and signed by the client. Take it to the court, and the court will not judge it.

But legally speaking, the relationship between creditor's rights and debts of loans can not be determined by an IOU. The existence of loan relationship can be proved by witnesses, bank transfer records and other parties, and an evidence system (commonly known as evidence chain) should be formed. In other words, I owe you a loan. But evidence is everything.