If at any time the amount of cash and investments held in the sinking fund exceed the proportionate amount required by section three hereof, the Sinking Fund Commission shall, if it deems it expedient so to do, have the right to credit such excess, or any part thereof, against the appropriation that would otherwise be required for the following year or years, and the next and succeeding annual amounts required to be appropriated shall be reduced accordingly.
53 P.S. § 22676