Try the Western Union at Santa Fe and Ayacucho, do not stay in line, go directly to the small door on
the right (if you have dollar), tell the guy inside to open the door for you, you can exchange your dollar
inside. It is safe. They have pretty much the blue rate. This place is not a tourist exchange place,
it's a low profile shop.
Many local people go there to pay bills, that's why you need to go inside.When you are done, walk out with
a calm look, check out the people in the room then exit to Santa Fe. Sometimes I exchanged over $1000,
I just walk out like walking out of a fruit store, be alert but do not look like a scared tourist.
Also do not dress like tourist, do not carry any type of bag, especially no back pack.