There's less currency risk than you seem to think here.

Let's suppose you price out a plane ticket on united.com with a Brazilian billing address (and thus a Brazilian ticketing city, fare structure, availability, and prices).

At some point in the purchase process, united.com will say "this plane ticket costs 2000 BRL". You'll accept that price and click "pay with this credit card."

Within 0-2 hours (usually), UA will ticket your itinerary and will charge your credit card 2000 BRL.

Assuming your credit card is billed in USD, on the day the transaction is posted, Chase will use the Visa interchange rate that was valid on the day of purchase (this rate is set daily and varies once a day), to convert your purchase price from BRL to USD. You'll usually be able to tell what rate Visa is going to use on a given day for your purchases; see http://corporate.visa.com/pd/consume...r_ex_rates.jsp for these rates for today and historically. Chase/Visa may slightly discount the rate, effectively baking in a bank interchange fee. (This is not the same thing as charging you an extra, visible transaction fee line item, which your United Explorer card does not do.)

It's not like Visa is gonna wait a month and then, when your bill comes due, say "ah, looks like you paid 2000 BRL for something, we're going to convert that at today's market rate" — they're not that perverse.

So when you pay for a BRL-denominated purchase with a US Visa card, you are not assuming a lot of currency risk here — (1) UA discloses the price in BRL right away and you can decide whether that is a good deal for yourself before you hit purchase; (2) assuming UA tickets on the day they quote the rate, the BRL-to-USD rate set by Visa is fairly predictable and should not be a surprise.

Barring unusual circumstances (paying for plane tickets in Argentina in person in cash), usually the reason you get a better deal from buying tickets with billing city in another country is NOT exchange rate arbitrage, but is instead DIFFERENT PRICES in different markets.