The employee should REQUEST that his employer 'dock' his pay this year in the amount of the repayment.

If the Employer will not, then the employee reports the repayment on Sch A line 16 in 2019 or 2020 and is generally screwed).

See Pub 17 on Repayments

If the amount you repaid was more than $3,000, you can deduct the repayment as an other itemized deduction on Schedule A (Form 1040), line 16, if you included the income under a claim of right. This means that at the time you included the income, it appeared that you had an unrestricted right to it. However, you can choose to take a credit for the year of repayment. Figure your tax under both methods and compare the results. Use the method (deduction or credit) that results in less tax.