(a) The commissioners court may invest money in a sinking fund accumulated for the redemption and payment of any bonds issued under this chapter in:
(1) bonds of the United States, this state, or a county, municipality, school district, or road district of this state;
(2) bonds of the federal Farm Credit System; or
(3) certificates of indebtedness issued by the secretary of the treasury of the United States.
(b) Sinking funds accumulated for the redemption and payment of bonds issued under this chapter may not be invested in bonds the terms of which provide for a maturity date after the date of maturity of the bonds for which the sinking fund was created.