The township assessor is entitled to compensation for services in the sum determined by the board of township supervisors for the time actually and necessarily employed in making and completing the assessment of the township and mileage at a rate not exceeding the allowable mileage rate accepted by the United States internal revenue service for each mile [1.61 kilometers] necessarily traveled in the performance of the duties of office. The compensation must be paid out of the township treasury upon an itemized statement setting forth the actual time spent in the work of assessor, approved by the board of township supervisors.