Answered by Shaykh Faraz Rabbani
I once borrowed a book from someone. I put the book on a university prayer room book shelf. After a while it disappeared. Many months later a book reappeared which looked almost exactly like the one that disappeared. I took it, thinking that it was the one that disappeared. Was it right for me to take the book? If not, can I tell somebody to put the book back on the shelf and do no more?
1. When you borrow something from another, it is a trust (amana), and one has taken on the commitment to safeguard it and not to be negligent.
2. When one is negligent about a trust, and it is lost or stolen as a result, one has to compensate for it, by replacing it or giving its owner its equivalent price.
3. Leaving another’s book in a public bookshelf is negligence.
4.When the book reappeared ‘many months later’, in your situation, you were to exercise your judgment.
It would not be permissible for you to pick up the book unless you are sure that it was in fact the very same book that you left there. The default would be that it is not.
Until you have clear signs (such as a name, unique damage, etc) that would make one reasonably sure about it being the very same book, it would be wrong to take it. As such, you would have to put it back.