Sec. 2. In making the annual per capita apportionment to the schools of the counties having a population of not less than 60,000 nor more than 73,000, the County School Trustees shall also make an annual allowance out of the State and County Available Funds for the payment of the salary of the Superintendent of Public Instruction not less than $2,800.00 nor more than $3,800.00 and office expenses in any sum not exceeding $200.00 per annum for stamps and stationery; and the Commissioners' Courts of the Counties having a population of not less than 60,000 nor more than 73,000 may expend out of the general funds of said counties any sums not exceeding the sum of $600.00 per annum to defray the expenses incurred by said County Superintendent which said sum shall be paid by said Commissioners upon certificate of said Superintendent that the expenses have been incurred in the discharge of his duties as such Superintendent.