5 USC 5535 - Extra pay for details prohibited

(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.
(b) An employee may not receive
(1) additional pay or allowances for performing the duties of another employee; or
(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.

This subsection does not prevent a regular and permanent appointment by promotion from a lower to a higher grade of employment.