22 U.S.C. § 3964

(a) The Secretary shall assign all Foreign Service officers and Foreign Service personnel (other than Foreign Service personnel who are paid in accordance with section 3967 of this title or section 3968 of this title ) to appropriate salary classes in the Foreign Service Schedule. (b) (1) The salary class to which a member of the Service is assigned under this section shall not be affected by the assignment of the member to a position classified under subchapter V. (2) Except as authorized by subchapter I of chapter 35 of title 5, changes in the salary class of a member of the Senior Foreign Service or a member of the Service assigned to a salary class in the Foreign Service Schedule shall be made only in accordance with subchapter VI. The Secretary shall prescribe regulations (which shall be consistent with the relevant provisions of subchapter VI of chapter 53 of title 5 and with the regulations prescribed to carry out such provisions) providing for retention of pay by members of the Service in cases in which reduction-in-force procedures are applied.