25 U.S.C. § 59
Transfer of funds for payment of employees; details for other service
When not required for the purpose for which appropriated, the funds provided for the pay of specified employees at any Indian agency may be used by the Secretary of the Interior for the pay of other employees at such agency, but no deficiency shall be thereby created; and, when necessary, specified employees may be detailed for other service when not required for the duty for which they were engaged.