Why did Greene have to make a deal with another crime boss?

Roth in his soliloquy to Michael in Havana, admitted Roth knew Greene was head-strong, talking loud, saying stupid things yet left Greene to turn up dead

Why didn't Roth intercede in this thing between the Corleones who were considered “all done” and Greene the kid Roth grew up with, loved and trusted who sorta looked up to Roth

Then Someone may not have put a bullet through Greene's eye Thanks! pal