I'm doing almost the exact same thing - which I also did a few years ago. I don't recall any challenges at the border crossing.
In fact, I intentionally scheduled it this way on the upcoming trip as there seem to be higher prices on one-way fares between Argentina and Brazil. As this is part of a broader trip, I only needed a one-way which was not pricing out so well.
So, I fly Buenos Aires to Iguazu and, when I'm done, head across the border via taxi and then fly from IGU to Rio availing myself of a cheap one-way ticket.