26 USC §3402. Income tax collected at source
( n ) Employees incurring no income tax liability
Notwithstanding any other provision of this section, an employer shall not be required to deduct and withhold any tax under this chapter upon a payment of wages to an employee if there is in effect with respect to such payment a withholding allowance certificate (in such form and containing such other information as the Secretary may prescribe) furnished to the employer by the employee certifying that the employee—
(1) incurred no liability for income tax imposed under subtitle A for his preceding taxable year, and
(2) anticipates that he will incur no liability for income tax imposed under subtitle A for his current taxable year.
26 USC §3402. Income tax collected at source
(p) Voluntary withholding agreements
(3) Authority for other voluntary withholding
The Secretary is authorized by regulations to provide for withholding—
(A) from remuneration for services performed by an employee for the employee's employer which (without regard to this paragraph) does not constitute wages, and
(B) from any other type of payment with respect to which the Secretary finds that withholding would be appropriate under the provisions of this chapter,
if the employer and employee, or the person making and the person receiving such other type of payment, agree to such withholding. Such agreement shall be in such form and manner as the Secretary may by regulations prescribe. For purposes of this chapter (and so much of subtitle F as relates to this chapter), remuneration or other payments with respect to which such agreement is made shall be treated as if they were wages paid by an employer to an employee to the extent that such remuneration is paid or other payments are made during the period for which the agreement is in effect.
