&gt; Argument for what?<p>An argument as to how restricting the use of cash (or eliminating the use of cash entirely) inherently conflicts with the first or fourth amendments.<p>&gt; A law that says &#x27;you may only contract with [insert privileged electronic currency here]&quot; or something similar.<p>The usual method would be more like &quot;All damages, including -- but not limited to -- those for breach of contracts specifying payment in some other currency, shall be awarded and assessed in [privileged currency] as of the date the debt became due and payable.&quot;<p>&gt; If the enforcement is through confiscation of other currencies it will likely violate 4th amendment protections on personal effects.<p>That&#x27;s a creative argument, but, no, Fourth Amendment search and seizure protections do not prohibit confiscation of contraband.<p>&gt; If certain contracts are banned (instead of, or in addition to, confiscation) it would violate speech protections.<p>Contracts that are contrary to public policy -- such as those whose fulfillment requires an act prohibited by law -- are generally void for that reason; this is a well-settled principal of law and is not contrary to the First Amendment. The idea that the First Amendment means <i>any</i> contract obligation, without restriction, is immune from prohibition is, well, silly in the extreme.