The board shall collect the principal and interest on the bonds and credit the amount collected to the general improvement fund, except that if the bonds are issued to acquire or to provide money for the purchase of certain outstanding bonds, it shall be used only for that purpose, and all of the money not so used and all sums received in payment of principal or interest on the bonds acquired by the county or received from the sale thereof shall be used for the payment of the principal and interest of the bonds issued pursuant to this article.