There's been many cases where the buyer didn't have legal access to get U$S (as most Argentinian's don't) and got to pay in pesos. But at what rate, depends on what's signed and the judge you get. This case they had agreed to pay "contado con liqui" if there ever was a problem to get US bills.
http://www.abogados....xtranjera/12614
http://www.abogados....xtranjera/12614