At the Eretz Hemdah Institue, where I work and am in charge of answering questions that we receive, we have written a response to a similar question (it hasn't been published yet), which addresses your question as well:
If a gazlan steals an item worth one zuz, and it goes up to four zuz, then he damages it, what is he liable for? On the one hand it left the reshut of the owner, on the other hand it was further damaged similar to the case of a gazlan who steals from a gazlan and is liable himself for the additional damage.
The Shulchan Aruch (Choshen Mishpat 354:3) rules: “One who stole an animal or a vessel, or similar items…[if] it was worth two at the moment of theft, but worth four at the moment of [the thief’s] being put on trial – whether he slaughtered, sold, broke the vessel, or lost it – he would pay as if it was at the moment of being put on trial. If the animal died or the vessel was lost from him, he would pay its value at the moment of theft.”
Similarly, the Shulchan Aruch rules (Ibid., 362: 10): “One who stole a barrel of wine from his colleague and it was worth a dinar at the moment of theft, but it went up in price while he possessed it and valued at four, and he broke the barrel or drank from it, or sold it, or gave it as a present after it went up in value, he would pay four like at the moment of ‘departure from the world’; whereas had he left it, it [the value] would return by itself. If it was broken by itself or got lost, one would pay a dinar, like the time of theft.”
The source of this ruling is in the Talmud, Tractate Baba Kama 65a: “One who stole a barrel of wine from his colleague: if it was originally worth a zuz, and later worth four zuz, if he broke it or drank from it, he would pay four; [however,] if it [the barrel] became broken by its own accord, he would [only] pay a zuz.”
The Rishonim debate – along with the Acharonim in their wake – the reason for the difference whether one had broken the item or if it became broken on its own accord.
The Nemukei Yosef (Baba Metziah 25a, in the pages of the Rif) and the Ketzot HaChoshen (34: 3) reason that after stealing an object, one is obligated only the value at the time of theft. Therefore, one cannot obligate him in the price rise that had occurred while the item was in his possession. However, if he broke the item, he would be considered a damager (mazik). Therefore, he would be obligated to pay its value at the time of damaging the item.
On the other hand, the Ritvah (Baba Metziah 43a, “amar Raba”) and the Netivot HaMishpat (34: 5) argue that any change that a thief makes to the item is considered further theft. Therefore, although he was previously considered a thief, at the moment that he broke the item, he would once again be considered the one who stole the item. Subsequently, he would be obligated at the moment of his breaking the item, as opposed to its becoming broken by itself – in which case, he would be obligated according to its original value.
The distinction between the Ketzot and the Netivot is that according to the Ketzot the additional chiyuv to pay is a chiyuv of mazik (damager), while according to the netivot the chiyuv is one of a gazlan. There are possible ramifications to this distinction and later Achronim discussed this issue at length. See also the wording of the gemara in Baba Metziah (43a) which appears more like the Netivot, and this appears to be the majority opinion.