There is a rematch clause in there for a reason.If Harrisson does some how beat Haye i think a Haye would disappear for a couple of months then resurface with a rematch date announced. It would be a massive blow to him and it would tarnish his reputation but life goes on and he would have to put it right in a rematch(which would still would make him millions)
Haye would rematch him and probably still go into that fight the favourite. If he won it though, there is no chance he'd fight a Klitschko.
He would blame personal issues/illness/problems with the training camp and take the rematch presumably, although Haye is someone that might give some credit if it was quality from Harrison. I can't see it happening though!