It's UFC's fault if this falls though. Luke has no obligation to still take the fight, so he has the negotiating power here. if UFC doesn't want to pay him an extra 50 or 100k or whatever the case may be, that's on them. You would be demanding more money too in this scenario if you were him.
Also taking the money out of Romero's purse creates problems too, if he can't win the belt and won't get paid then Yoel may no longer want to fight either. It really is down to UFC to shell out money to fix this problem, not pit the fighters (and fans) against eachother.