I'm not sure if you are referring to the documentary, but, while he doesn't explicitly refer to it as a broken promise, he basically makes it seem like Lucas planned for him to do the voice and then it was budgetary concerns and oversights that led to him not doing so. He pretty explicitly says that they didn't get around to overdubbing him while they were in England (makes it sound like Lucas forgot about it) and couldn't afford to fly him over to America to do the overdubbing. He then goes on to state that, luckily they got a really great "voice actor", JEJ, to do the work in America – which I thought was more than a bit of a slight to refer to James as a voice actor given his career both before Star Wars and after.