All accrued interest received on the sale of the bonds shall be placed in the fund to be used for the payment of the principal of, and interest on, the bonds, and the remainder of the proceeds of the bonds shall be placed in the treasury of the authority and applied to

secure the bonds or for the purposes for which the debt was incurred. However, when the purposes have been accomplished, any money remaining shall be either (a) transferred to the fund to be used for the payment of principal of, and interest on, the bonds or (b) placed in a fund to be used for the purchase of the outstanding bonds in the open market at prices and in the manner, either at public or private sale or otherwise, as determined by the authority. Bonds so purchased shall be canceled immediately.