Hm, I feel like this is way less sinister than it sounds. From what I know of his prior reputation, Will Happer has long made his position on climate change clear, so it's not like he changed his mind when people offered him a big pile of money. If you already believe something and someone is going to give you money to promote your views, why on earth would you say no?
You're missing the point.
It's not that this guy was paid off and therefore changed his views.
It's that he published research that he claimed was independent that was not, and in the process neglected to disclose his sources of revenue and conflicts of interest.
Those very conflicts of interest would, under normal circumstances, force folks to question the nature of the research and the quality of the results. IMO that's getting awfully close to academic misconduct. It's certainly extremely unethical.
Edit:
BTW, "Merchants of Doubt" has covered this territory extensively, so to be honest, I'm not sure Greenpeace has added much to the dialog, here, aside from flat out catching folks in the act.