D.C. Code § 1-503

No civil officer, clerk, draftsman, compensation messenger, assistant messenger, mechanic, watchman, laborer, or other employee shall, after June 30, 1905, be employed in any office, department, or other branch of the government of the District of Columbia or be paid from any appropriation made for contingent expenses, or for any specific or general purpose, unless such employment is authorized and payment therefor specifically provided in the law granting the appropriation or is authorized as hereinafter provided, and then only for services actually rendered in connection with and for the purposes of the appropriation from which payment is made and at the rate of compensation usual and proper for such services, and on and after July 1, 1905, all moneys accruing from lapsed salaries, or for unused appropriations for salaries, shall be covered into the Treasury as are the balances of other unexpended appropriations for the support of the government of the District of Columbia.