Terrance Howard has altered his story a time or two. One site I was at examined the changes he's made over the years.
I actually liked him in the role. For continuity sake wish he'd stayed but from what I can discern is he, his agent also perhaps, bear some responsibility for what happened. Marvel has a history of going cheap so I don't see them as blameless but Howard was not the selling focus of the film. Legally, the 'i's and 't's were such that it appears how things were handled were legit, just not to Howard and his agents liking cause it structured his pay downward.
Downward is what they got Don Cheadle for and rest is history, even if Howard would like to keep editing it in a more negative way every time asked.