The Argentine immigration authorities will NEVER ask to see a return ticket, proof of funds, proof of lodging, etc from a US passport holder. In fact, as long as you fill out the immigration entrance form correctly, and have paid the reciprocity free (for non residents) they will ask no questions at all and there will be no verbal communication except maybe hello and thank you. The Brasilian federal police (who administer immigration) may ask how long you are staying but again, as long as your visa is in order, will NOT ask for a return ticket, proof of funds, proof of lodgings from US passport holders. Since September 2010 (when the US and Brasil came to an agreement to extend visa duration to 10 years and to stop busting each other citizens balls at the immigration stations) getting a Brasilian visa and entering the country has been a piece of cake.