What is the ruling of Islamic law regarding a brother who transferred ownership of a chalet to himself using a general power of attorney without the knowledge of his partner, and refused to pay the agreed-upon value of the partner's share?

As for the rights, the lawsuit against the brother and the ongoing investigation into it are in progress.

As for what is permissible and what is forbidden, the impermissibility of lying and consuming wealth unjustly is clear. Exploiting a general power of attorney to steal the client’s money is deception, treachery, betrayal, and fraud, and it exposes its perpetrator to disgrace and torment, for the Prophet, peace and blessings be upon him, said: "When Allah gathers the first and the last on the Day of Resurrection, a banner will be raised for every betrayer, and it will be said: This is the betrayal of so-and-so, son of so-and-so" (Narrated by Muslim), and he also said: "He who deceives us is not of us, and cunning and deceit are in the Fire."