31 U.S.C. § 3111: New issue used to buy, redeem, or refund outstanding obligations

An obligation may be issued under this chapter to buy, redeem, or refund, at or before maturity, outstanding bonds, notes, certificates of indebtedness, Treasury bills, or savings certificates of the United States Government. Under regulations of the Secretary of the Treasury, money received from the sale of an obligation and other money in the general fund of the Treasury may be used in making the purchases, redemptions, or refunds.