In the event the gross value of petroleum or other crude or mineral oil, natural gas, casinghead gas or liquids extracted therefrom is adjusted downward by any agency of the United States of America or a court of competent jurisdiction adjudicating an appeal from said agency, then the amount of the tax paid in excess of the tax due on the adjusted gross value shall be considered excess tax. Within one (1) year following the final determination of the gross value, any producer or purchaser who has paid any such excess tax may apply for a refund, and the Tax Commission, upon proper finding, shall have the authority to refund the amount of excess tax paid. Any refund may, at the discretion of the Tax Commission, be made in the form of a credit against future tax payments.