If there is money in the state treasury to the credit of the sinking fund not required for the payment of the interest on the bonded debt of the state, the board of commissioners of the sinking fund shall apply it to the payment of any part of such debt, on such terms as the board deems for the best interests of the state. If no such debt can be paid, the board shall invest such money in certificates of the bonded debt, not due. After such investment, if such moneys or any part thereof are required for the payment of any part of the bonded debt, maturing thereafter, the board may sell and transfer such certificates and apply the proceeds thereof to such payment.