Actually, you may find multiple options for this. When I had just moved to BsAs, I used Banco Piano as it offered the best rate among the 4 options that I had in Centro. They do require you to carry your passport as a valid reason for your presence in Argentina and the need to exchange money.I prefer to use a bank. But if no bank will exchange dollars for pesos unless you have an account with them is there a non-bank exchange place near Austria and Las Heras?
I can affirm from personal experience that Banco de la Nation will indeed exchange dollars without an account - and the line was long. Otherwise no problems, just show DNI or passport.