She belongs to Yang Kai, so he should just take her back from whoever happens to buy her, as it would be an invalid transaction anyway. The only problem is that he might not have enough power for that. But he most certainly won't have enough coin either. That being said, if he took out the furnace, wouldn't she just dive back into it? What other proof would he need to show whom she actually belongs to? The problem in this approach is, the powerful people wouldn't care and they would actually rob the furnace from Yang Kai. Which pretty much means he just needs to get strong enough to take her back using his own power, either sneakily or by leaving one or two mansions in ruins.