5 U.S.C. § 5535

1. (a)An officer may not receive pay in addition to the pay for his regular office for performing the duties of a vacant office as authorized by sections 3345–3347 of this title.
2. (b)An employee may not receive—This subsection does not prevent a regular and permanent appointment by promotion from a lower to a higher grade of employment.
   1. (1)additional pay or allowances for performing the duties of another employee; or
   2. (2)pay in addition to the regular pay received for employment held before his appointment or designation as acting for or instead of an occupant of another position or employment.