20 C.F.R. § 404.1045 Employee expenses.
Title 20 - Employees' Benefits
Amounts that your employer pays you specifically—either as advances or reimbursements—for traveling or for other ordinary and necessary expenses incurred, or reasonably expected to be incurred, in your employer's business are not wages. The employer must identify these travel and other expenses either by making a separate payment or by specifically stating the separate amounts if both wages and expense allowances are combined in a single payment.
Title 20: Employees' Benefits
PART 404—FEDERAL OLD-AGE, SURVIVORS AND DISABILITY INSURANCE (1950– )
Subpart K—Employment, Wages, Self-Employment, and Self-Employment Income
Wages
§ 404.1045 Employee expenses.