Team medical didn't pick up on this? I would be surprised if there wasn't a clause somewhere in that record deal contract to deal with reduced payments for things such as this. Surely this injury was know before you signed him.
From what I gather, the injury turned out to be worse than anybody thought (even the surgeons), so I don't think Fulham cheated Hull, it's just bad luck. I don't think there's any way to assess ligament damage accurately without arthroscopic surgery, so it's not something the team medical could have picked up on.
Best of luck to him though, hopefully he's back on the pitch next season.
Speaking of injuries, it seems that Eduardo tried a little too hard to impress on his debut as he's out for two weeks with a hammy strain.