In theory, we might be able to derive an answer from a different scenario. At the heart of the question here is if someone damages/steals but the owner in the end turns out to have gained, or at least not lost, due to his damage, is the person still liable.
A somewhat similar case is a situation that someone damaged or stole an object that was insured. The owner did not incur a monetary loss (he may of even gained sometimes) from the damage that was done. More so, through the damage he received an insurance check. Is the damager liable?
Although there’s a machlokes about insurance, the dispute centers on the exact mechanics of insurance payments, whether they are tied to the object or to the one who paid premiums. Regardless, this case would be similar in theory to those who look at insurance as a side benefit tied to the owner. The Maharsham was one of those held of this theory in insurance, and in Teshuvos Maharsham 4:7 he says that if the insurance paid for the damages, the one who damaged is still liable to pay since the insurance is a separate thing.
So it seems that if someone caused damage or stole, and the owner had a side benefit from that, the person is still liable for his actions. Even those who disagree by insurance it is only because they hold the item is insured not the owner, and so it’s not really counted as a damage. But in a case where the benefit is from a side thing, all seem to agree that the person is still liable for the damages. So in your case, any side benefit the owner received (item was saved from fire etc) shouldn’t have any bearing on the liability of the damager.