"Paper currency, in the form of the Federal Reserve Note, is defined as an "obligation[] of the United States" that may be "redeemed in lawful money on demand." 12 U.S.C. § 411 (2002). These bills are not "money" per se but promissory notes supported by the monetary reserves of the United States."
http://ftp.resource.org/courts.gov/c...0.02-2288.html
Quoted text is in the second to last paragraph at link.