I assume you are traveling on business on a business visa, otherwise you must get a Brazil tourist visa ($100) - takes 4-7 days from Brazil Embassy in DC or local consulate. You can get money from an ATM machine. There is also an American Express money exchange on the second floor in GRU - above UA checkin area.

Be advised most people do not speak English. I suggest you type up the name and address of your hotel so you can show a cab driver. (Cab drivers do not speak English). I would recommend a good secure hotel. At most hotels only a few employees speak English.

I would not use a credit card except at hotels. Many report their credit card number being used by others later.