Ha, that would be pretty funny to see that done.
At first thought, I would say that those guys were right. But, I don't think the NFL would allow a contract like that. There has to be some sort of protocol for those incentives.
Plus, the player wouldn't want it either. What happens if the player blows a knee in the pre-season? He wouldn't get the 5 yards and would lose himself 7 million dollars...not much he could do about it either.