They wouldn't preemptively prohibit that contract, but if it went to an American court the court's decision would be in USD. If it went to an Argentinian court, it sounds like the court's decision would be in terms of whatever currency was specified in the contract (although, again, I'm not clear what exactly the new Argentinian policy does).

I'm saying that "the one you can always settle your debts in (though that's not universal)" is actually the most accurate description of legal tender that you gave.