Louisiana Revised Statutes Tit. 26, § 422. Exemption in favor of United States

The taxes levied in this Chapter shall apply to sales to the United States or any agency or department thereof except any sales in which the imposition of the tax is prohibited by any federal law.  When any tax is paid by the United States, or any agency or department thereof, which imposition of such tax is prohibited by federal law, it shall be refunded by the secretary upon proper showing and authentic proof satisfactory to him that the purchase was bona fide and the tax was paid.  This refund shall be paid from the funds in the hands of the secretary which have been collected under this Chapter and not paid over to the state treasurer.