Section 23. Except as otherwise provided in section twenty-four of this chapter and in section fifteen of chapter two hundred and seventy-six, district attorneys and assistant district attorneys shall receive for traveling expenses necessarily incurred in the performance of their official duties such sums as shall be approved by the district attorney, to be paid by the commonwealth. In each district the traveling and contingent expenses incurred by the district attorney, or his designee, while attending conventions that have been convened for the purpose of considering questions of law reform or statistics and other legal matters affecting the welfare of the people, and the membership dues of the district attorney, or his designee, in the National District Attorney Association, shall, subject to appropriation, be paid by the commonwealth, upon receipt of a certificate from the district attorney that they were necessarily incurred under the provisions of this section, and upon the approval of the district attorney.