Hi, welcome to Spider ID. 🙂 You have a Huntsman Spider of some-sort. It looks enough like this Polybetes that I think that is the genus. I need to do a little reading to see if it can be narrowed down further.
Edit: Note to admin., genus needed for Polybetes.
I can’t be sure but from looking at a distribution map (Gerschman de P., B. S. & Schiapelli, R. D., 1965b) but I see a data point for Polybetes punctulatus right over Buenos Aires, there are other similar possibilities nearby however like P. pythagoricus which is more common.