Ibn Taymiyyah’s view that a decline in market price is recoverable from the usurper?

The answer is no, for the decline in price here is due to a diminution of the item itself, not merely to its value. Thus, it does not refute those who maintain that a loss caused by a fall in market price is not compensable. However, we have previously explained this issue in detail: if he deliberately delays returning it until the season ends and the price falls, then he is liable; otherwise, he is not.

His statement, “If it turns into vinegar,” rather than, “If it is made into vinegar,” is because wine, if it turns into vinegar on its own, is lawful, since this occurs by the act of Allah, not by our action. But if it is made into vinegar by our action, it is unlawful, based on the hadith of Anas ibn Malik, who reported that the Prophet was asked about wine being made into vinegar, and he said, “No” (1). This is also because the Lawgiver commanded that wine be poured out (2). Had it been lawful to make it into vinegar, He would have commanded that it be made into vinegar, since making it into vinegar preserves it, whereas pouring it out destroys it. If it could lawfully regain its status as property, the Lawgiver would have directed people to do so. Since He did not direct them to it, it is known that making wine into vinegar is unlawful.

However, if someone who believes that making it into vinegar is lawful—whether Muslim or non-Muslim—does so, does it become lawful?

The correct view is that it does become lawful, because it has turned into vinegar in a permissible manner and therefore becomes permissible. Accordingly, vinegar imported from non-Muslim lands is lawful for Muslims, even if it was made into vinegar through human action, because it was made into vinegar by someone who considers this permissible.