31 U.S.C. § 5115: United States currency notes

(a) The Secretary of the Treasury may issue United States currency notes. The notes—(1) are payable to bearer; and (2) shall be in a form and in denominations of at least one dollar that the Secretary prescribes. (b) The amount of United States currency notes outstanding and in circulation—(1) may not be more than $300,000,000; and (2) may not be held or used for a reserve.