D.C. Mun. Regs. tit. 9, r. 9-138
Amounts, in addition to taxes required to be withheld, may be withheld by an employer under an agreement between the employer and employee.
Additional amounts withheld pursuant to an employer-employee agreement, together with the amounts otherwise required to be withheld, shall be considered the amounts required to be deducted and withheld.
The additional withholding, together with the amounts otherwise required to be withheld, shall be reported on Form D-2 as income tax withheld from wages.
The agreement shall be in writing in the form prescribed by the Deputy Chief Financial Officer, and shall be executed in duplicate by the employer and the employee. One (1) copy of the agreement shall be retained by the employer and one (1) copy shall be retained by the employee.
An agreement for additional withholding shall be effective for the period as may be mutually agreed upon; however, unless the agreement provides for an earlier termination, either the employer or employee may terminate the agreement by giving written notice effective with respect to the first payment of wages made on or after the first status determination date (January 1st or July 1st), which occurs at least thirty (30) days after the date on which the notice is given.
D.C. Mun. Regs. tit. 9, r. 9-138