If upon a final disposition of the issues involved in a proceeding mentioned in § 56-242, the rates or charges as finally determined by the Commission, or the court having jurisdiction of the subject matter, are in excess of the rates and charges prescribed in any temporary order issued in such proceeding, then such public utility shall be permitted, over such reasonable time as the Commission shall fix, to amortize and recover, by means of a temporary increase over and above the rates and charges finally determined, such sum as shall represent the difference between the gross income obtained from the rates and charges prescribed in such temporary reduction order, and the gross income which would have obtained, during the period such temporary reduction order was in effect, based upon the same volume, from the rates and charges finally determined.