You will need Argentinean Pesos for:
1. The climbing permit, if you don't want to pay for it in US dollars
2. Local gear hire and purchases, including VHF radio rental and white gas purchase
3. Local expenses in Mendoza and Penitentes, e.g. bottled water, food and drinks, last minute gear purchases in the Mendoza shops
4. Taxi fare to return to the airport at the end of your trip
It is best to buy only the amount of pesos you need - the peso is worthless outside of Argentina. Remember that in Mendoza most hotels, shops and restaurants will accept payment by credit card. On the mountain the preferred currency is US dollars. Food, drinks, showers, etc can be purchased from the different support companies at the base camp on both sides of the mountain and at Confluencia camp on the Horcones Valley (Normal) route.
In December 2017 at local Mendoza banks the exchange rate was around 19 pesos to the US dollar. A climbing permit cost 15560 pesos or 818 dollars at 19:1 peso/dollar exchange rate. Instead of paying for the permit in pesos I decided to buy it using US dollars. This can be done at the Pago Facil next to the ministry of Tourismo in the centre of the town. I was expecting to pay more but in-fact the price of the permit was 800 dollars, so I saved 18 dollars paying for the permit in dollars rather than exchanging the money at a bank and then paying in pesos.
Outside the banks you will see groups of men that discreetly say "exchange" to you and you walk past. This is the active black market. As I needed pesos in the afternoon when the banks were closed I exchanged 200 dollars with one of these guys. When you indicate that you what to exchange some money they take you to a small shop nearby where the transaction is done. Expect to haggle with them, i.e. don't accept the first rate they offer. When the banks are open the rate the blackmarket guys offer improves to slightly higher than the rates the banks will give you. When the banks are closed the rate goes down, below the bank rates as they know you need the cash quickly, as I did. I got a rate of 18.7 pesos to the dollar but I was only exchanging 200 dollars with them so I only lost 60 pesos or about 3 dollars.