5 U.S.C. § 5536 - U.S. Code - Unannotated Title 5. Government Organization and Employees § 5536. Extra pay for extra services prohibited

An employee or a member of a uniformed service whose pay or allowance is fixed by statute or regulation may not receive additional pay or allowance for the disbursement of public money or for any other service or duty, unless specifically authorized by law and the appropriation therefor specifically states that it is for the additional pay or allowance.