From Default of the Fourth Liberty Bond:

The legal basis for the refusal of the US Treasury to redeem in gold was House Joint Resolution 192, dated June 5, 1933.[18] This resolution was later held to be unconstitutional and thrown out by the U.S. Supreme Court.[19] Chief Justice Hughes writing for the majority elaborated the precedent that Congress may not legally nullify its own contracts:

We conclude that the Joint Resolution of June 5, 1933, insofar as it attempted to override the obligation created by the bond in suit, went beyond the congressional power.
?Chief Justice Charles Evans Hughes, Perry v United States, 294 US 330 (1935), Page 294 U. S. 354

Due to the significant restrictions placed on gold trading by Roosevelt's reforms, the Court ruled that the bond-holders' loss was unquantifiable, and that to repay them in dollars according to the 1918 standard of value would be an "unjustified enrichment".[17] The ruling therefore had little practical effect.