Section 94. The reasonable expenses, including supplies and equipment, incurred by probation officers of the superior court and the probate court in the performance of their duties shall be approved and apportioned by the court, and paid by the commonwealth. Such reasonable expenses shall include the traveling expenses necessarily incurred by such a probation officer in connection with attendance at sessions of said court outside of the town in which the principal office of such probation officer is maintained, such expenses to be computed from and to said town. Money to be used for the necessary expenses to be incurred by such a probation officer in going outside the commonwealth for the purpose of bringing back for surrender to the court a person who is on probation shall be advanced by the treasurer of the commonwealth, upon presentation of a certificate signed by the probation officer and approved by said court. After his return such probation officer shall account for such money by filing with said state treasurer itemized vouchers, duly sworn to, approved by the court, setting forth the necessary expenses so incurred and any unexpended balance of such money shall be paid to said state treasurer. Subject to section eighty-one of chapter two hundred and eighteen, probation officers of district courts and of the Boston, Worcester, Bristol county and Springfield juvenile courts shall be reimbursed by the commonwealth for their actual disbursements for necessary expenses incurred while in the performance of their duties, including their reasonable traveling expenses in attending conferences authorized by section ninety-nine, not exceeding four hundred dollars to each in any one year, upon vouchers approved by the court by which they are appointed.