Employees Paying Back Signing Bonus - ADP/ACP Testing

An employer hires an employee in 2021 and pays a $10,000 signing bonus.  Two months after beginning employment, Employee enters 401(k) Plan.  In 2022, Employee terminates service and, under terms of employment contract, pays back sign on bonus by issuing a personal check in 2022.

Does this reduce that employees compensation in 2022 for purposes of Testing, if using the definition of Compensation under 414(s)?  I am inclined to say no, as you simply look at what the Employer paid the Employee during 2022.  Does this require new testing for 2021?

No and no.

It's similar, I think, to an employee repaying a distribution overpayment from a prior tax year. IRS position is that the amount received (or made available) is taxable (compensation in this case) for such year and that anything that happens in a subsequent year does not change that.

The person also received $X in taxable compensation from the employer before terminating which I think is not affected by cutting a personal check back to the employer. The person may be able to get some sort of personal miscellaneous tax deduction for the repayment (subject to those rules) but that isn't your concern, it's a matter for this person and his/her accountant.