26 U.S.C. § 4293: Exemption for United States and possessions

The Secretary of the Treasury may authorize exemption from the taxes imposed by section 4041, section 4051, chapter 32 (other than the taxes imposed by sections 4064 and 4121) and subchapter B of chapter 33, as to any particular article, or service or class of articles or services, to be purchased for the exclusive use of the United States, if he determines that the imposition of such taxes with respect to such articles or services, or class of articles or services will cause substantial burden or expense which can be avoided by granting tax exemption and that full benefit of such exemption, if granted, will accrue to the United States.