If he is fighting at the weight why the rehydration clause? It's unfair.
I feel two ways about it. Sometimes....sometimes, some fighters come in HUGE on fight night and it definitely gives them a huge advantage. I just can't picture a guy coming in at damn near 20 lbs heavier when the guy you're fighting only put on 10 lbs or less overnight.
On the other hand (as you mentioned), if they're fighting at a contracted weight, then you should be down to fight regardless of how much weight the other man has put on.
Is it Fighter A's fault that Fighter B won't just move up in weight because they're struggling to cut weight just to have advantage?