41 CFR § 301-52.22
Will any late payment fees I receive be reported as wages on a Form W-2?
June 9, 2020
No, the Internal Revenue Service (IRS) has determined that the late payment fee is in the nature of interest (compensation for the use of money). Your agency will report payments in accordance with IRS guidelines.
[FTR Amdt. 90, 65 FR 3056, Jan. 19, 2000]