Usurpation means that somebody takes possession of the property of another person unjustly and without his permission.
Usurpation is one of the major sins, and on the Day of Judgement, the person who has usurped will have to face a great punishment.
Issue 425: If a person usurps something, not only has he committed a Haram act, but he must return that property to its owner, and if he loses that thing, then he must repay that which was lost.
Issue 426: If the thing that was usurped becomes spoiled, then one must give the equal value of that item.
Issue 427: If the thing which was usurped has changed, such that it is better than it was in the beginning, for example, a bike (which was broken) and then is repaired, then if the owner of that property says that he wants it back just as it is, the usurper must give it to him, and is not allowed to take money for the trouble that one had gone through (to repair it), and one does not have the right to change the thing back to how it was in the beginning.