What is the legitimate Islamic solution that guarantees the buyer's right and absolves him of sin after discovering that the seller is not the original owner of the usurped apartment, and is it permissible for him to sell the apartment and take the purchase price, paying the excess to the original owner?

Whoever buys a usurped item becomes like the usurper in terms of guarantee, and he can seek recourse from the usurper for the price. Whoever finds the exact usurped item in someone's possession has a greater right to it, and the buyer can seek recourse from the seller for the price. The usurper's sale of the usurped item is invalid because he is selling what he does not own. It is not permissible for the buyer to sell the apartment to someone else, because it did not enter into his ownership through a valid sale. The wronged person should be patient and seek reward from Allah, and he should pursue legitimate means to reclaim his right, such as demanding to file a lawsuit against the usurper. Reaching a settlement with the original owner by handing over the apartment in exchange for the owner paying what the buyer gave to the usurper is the best option, and the payment of money by the original owner would be considered a charitable donation.