Appropriations now or hereafter provided for the Immigration and Naturalization Service shall be available for payment of (a) hire of privately owned horses for use on official business, under contract with officers or employees of the Service; (b) pay of interpreters and translators who are not citizens of the United States; (c) distribution of citizenship textbooks to aliens without cost to such aliens; (d) payment of allowances (at such rate as may be specified from time to time in the appropriation Act involved) to aliens, while held in custody under the immigration laws, for work performed; and (e) when so specified in the appropriation concerned, expenses of unforeseen emergencies of a confidential character, to be expended under the direction of the Attorney General, who shall make a certificate of the amount of any such expenditure as he may think it advisable not to specify, and every such certificate shall be deemed a sufficient voucher for the sum therein expressed to have been expended.