Argentina and Venezuela are the two countries in South America where they give an “official rate” for exchanging money at banks and ATMs and there is a better black market rate done informally with cash. What this means is that you are penalized for using ATMs at banks or paying for anything with a credit card in those two countries.
If you google “Argentina blue dollar” you will get a full explanation of the benefits of exchanging money on the black market in Argentina.
What this means to you is that ATM and credit cards will give you the official exchange rate of 8.85 Argentine pesos to the dollar, but if you have U.S. crisp hundred dollar bills, you can exchange them for 12.40 Argentine pesos to the dollar from a private money changer.
So say you go to an ATM in Buenos Aires and use your debit or credit card to get out 200 dollars worth of Pesos. The cash machine will spit out 1770 pesos. But if you go to the money changer dude on the corner near the central square and hand him 2 hundred dollar bills he will give you 2480 Argentine pesos. A much better deal. They prefer crisp hundred dollar bills. The exchange for 20s and 50s is slightly less. Don’t ask me why.
One other thing that many first time travelers don’t hear about is the importance of bringing crisp bills from your home bank to exchange in Latin America. No slight imperfections or tiny tears. They won’t take them.
Funny story, Albert the Scotsman who owns the hostel in Medellin where I stayed last year had a ripped 100 dollar bill that he hadn’t been able to use for years. I exchanged a fresh one and took his ripped one home where of course any bank in the States will exchange for a fresh one free of charge.
When I was in Argentina last year I exclusively used cash since your money goes 50% further. Where do you find black market money changers? Usually they hang out near the town square in the city center of medium to large towns. Or you can stop in at a hostel and ask the gringo backpackers on a budget where the nearest good money changer is. They will know. Small towns off the gringo trail won’t have them.