Is it permissible for me to benefit from the money that my brother continued to receive after leaving his job, and then gave it to me?

Your brother is not permitted to use the salary deposited into his account by mistake, nor any direct payments he received from it. Rather, he must inform his employer about it. If they grant it to him, then it is a gift to him. Otherwise, he is obligated to return it. You are also not permitted to benefit from it if your brother gives it to you. Instead, you must inform his employer of the matter, because whoever takes from an aggressor is like him. Scholars have stipulated that whoever takes from a usurper, a thief, or an aggressor, while knowing of their aggression, is like them. Al-Ulaish mentioned that anything that is intrinsically unlawful and is in the possession of a usurper or thief, it is not permissible to buy it from him, sell it, eat it, wear it, or accept it as a gift. And whoever does so is like the usurper.