If a spending unit purchases or contracts for commodities or services contrary to the provisions of this article or the rules and regulations made thereunder, in any material respect, as determined by the director, such purchase or contract shall be void and of no effect. The spending officer of such spending unit, or any other individual charged with responsibility for the purchase or contract, shall be personally liable for the costs of such purchase or contract and, if already paid out of state funds, the amount thereof may be recovered in the name of the state in an appropriate action instituted therefor: Provided, That the state establishes by a preponderance of the evidence that the individual acted knowingly and willfully.