At the end of the period of insurance, an adjustment of premium shall be made according to the actual expenditures of wages. The Commissioner of Finance shall compare the payroll of each employer paying premiums in accordance with this chapter for such calendar year with the payroll of the preceding year on the basis of which the premiums were assessed, levied and collected. If the payroll for the year during which the insurance was effected was greater than that of the previous calendar year, the Commissioner of Finance shall assess, levy and collect additional premiums in the same manner and on the same basis as the original premiums were assessed, levied, and collected. If the payroll is less than that of the previous calendar year, the Commissioner of Finance shall refund from the Government Insurance Fund the proportion of the premiums corresponding to the difference between the actual payroll for the year during which the insurance was effected and the year on the basis of which said premiums were assessed, levied, and collected; provided, however, that where the employer continues to employ workers affected by this chapter, the Administrator shall apply any excess premium to the payment of the premium due for the ensuing calendar year.