Delta is always going to return in the point of sale currency which in this case was Argentina Pesos... I know that sounds unfair and (in the case of Argentina pesos) extreme for your use case, but think about it the other way. I book a Euro-originating flight and pay in Euros. Euro appreciates against the dollar by 10% over the 9 month window. Would it be fair to the person in Europe to get fewer Euros back because it appreciated against the dollar? You're essentially asking Delta to run a foreign exchange bank for you.