All perfect praise be to Allah, The Lord of The Worlds, and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
Taking possession of people`s property unlawfully is among the major sins, for Allah, The Almighty, said about it: "And do not eat up your property among yourselves for vanities, nor use it as bait for the judges, with intent that ye may eat up wrongfully and knowingly a little of (other) people’s property"{Al-Baqarah/188}.
Moreover, Prophet Mohammad(PBUH) stated: "He who took a span of earth wrongly would be made to wear around his neck seven earths on the Day of Resurrection."{Bukhari}.
It is imperative that the usurper gives the above land to its rightful owner since he(usurper) has committed a sin which he must undo immediately since The Messenger of Allah (PBUH) said:" None of you should take the property of his brother in amusement (i.e. jest), nor in earnest. If anyone takes the staff of his brother, he should return it."{Abu Dawoud}. He is also obliged to remove that building and compensate the rightful owner for any damages.
It was stated in {Moghni al-Mohtajj, 3/362}: "Any addition to the usurped land, be it a structure or planted trees, must be removed and compensation must be paid against damages. The usurper is also liable to pay the benefits{rent and the like}of a property similar to it throughout the period of his usurpation.
Moreover, it is permissible for the usurper and the wronged person to agree on a certain solution: keeping the building as it is in return for a compensation, or selling the land to the usurper. However, it is the right of the original owner to take back his land as it was. And Allah knows best.