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.
It took me 3-16 minutes varied across multiple visits. In and around Florida/San Martin y Sarmiento/Av. Corrientes, you should be able to get your work done.
Maybe I am wrong of course but it seems less likely to me that I will get a bad bill at a bank. Also bank branches are so easy to find. Exchange places, in my experiences, not so much. Anyway ... yesterday I tried many bank branch offices as I walked down a stretch of Las Heras. Not a one would exchange cash for a non-client. So I wandered my way to an unofficial exchange place I have used several times in the past near the intersection of Ortiz y Cerviño (on Ortiz next to the Freddo).
I would only go into an Argentine bank of I had too. They are a major pain in the ass. A legitimate casa de cambio would be my choice. Before the blue dollar went away, I used western union. Never got a fake bill.
I lived there last year for 3 months and didn’t have a bank account but went to the Juan Justus branch of Banco Supervielle several times to cash dollars to pesos. No problem.....just bring your passport! No lines either! Not sure how close you are to there?
I mentioned earlier in this thread that I used a cambio near the intersection of Ortiz y Cerviño (on Ortiz next to the Freddo). The last time I was there I picked up their business card which says - address: Scalabrini Ortiz 3205; phone numbers: 4806-0094 / 4806-0504 / 4808-0049; horario: Lunes - Viernes de 9 a 17 hrs / Sabados de 10 a 13 hrs.