Is it permissible to demand that a trustee who stole what was entrusted to him return the stolen amount, or part of it, after verifying the truth of his claim that he was robbed, and would that be considered oppression or unlawful (haram)?

If the entrusted money is stolen without negligence or dereliction in its safekeeping, the trustee is not obligated to guarantee it, and the owner is not permitted to compel him to pay anything from it. However, if the theft occurred due to negligence and dereliction, the trustee is obligated to guarantee it, and the owner may demand the full amount from him. Dereliction would be placing the money in an unsuitable location for its safekeeping, for dereliction and shortcoming in trusts necessitate guarantee.