the things we do for miles and status...

I'm DC-based and have been searching hard for a UA/*A ticket to EZE under a 1000usd (which i guess is my threshold for a reasonable price.) anyhow, it seems that UA is matching some fares out of MIA (Aerolineas, LAN, AA) and I managed to grab a fare from MIA for 830 a/i after applying a 10% e-cert. Certainly not great, but the best I could seem to get...esp. if I want to get miles out of this.

My question here concerns my return journey (EZE-IAD-MIA). I am planning to just skip the last segment to MIA and just go home from Dulles...Do I risk losing out on all miles from my return trip if I do that? Or would I only be out the IAD-MIA miles?

Thanks for your responses.