Section 20. No account or demand requiring the certificate of the comptroller or warrant of the governor shall be paid from an appropriation unless it has been authorized and approved by the head of the department, office, commission or institution for which it was contracted; nor shall any appropriation be used for expenses, except gratuities and special allowances by the general court, unless properly approved vouchers therefor have been filed with the comptroller. No such voucher shall be submitted by such head nor shall any such approval be given by such head unless sufficient funds are allotted for such purposes at the time the voucher is submitted or the approval is given.