(a) The Secretary of the Treasury, under regulations prescribed by the President, shall enter into an agreement with the Mayor of the District of Columbia within 120 days of a request for agreement from the Mayor. The agreement shall provide that the head of each agency of the United States shall comply with the requirements of subchapter II of chapter 15 of title 47, District of Columbia Code, in the case of employees of the agency who are subject to income taxes imposed by that subchapter and whose regular place of employment is within the District of Columbia. The agreement may not apply to pay of an employee who is not a resident of the District of Columbia as defined in subchapter II of chapter 15 of title 47, District of Columbia Code. In the case of pay for service as a member of the armed forces, the second sentence of this subsection shall be applied by substituting “who are residents of the District of Columbia” for “whose regular place of employment is within the District of Columbia”. For the purpose of this subsection, “employee” has the meaning given it by section 1551c(z) of title 47 , District of Columbia Code.