What is the ruling on money received by the petitioner from some clients without prior stipulation, in exchange for facilitating certain transactions, and must this money be given out if it has intermingled with his own and he has purchased a car with it?

It is not permissible for an employee to accept a gift from someone who has dealings with the institution where he works, whether this was stipulated or not. This is based on the Prophet, peace and blessings be upon him, who said: "Gifts to officials are ghulūl (illicit gain)." And his saying concerning Ibn al-Lutbiyyah: "Why did he not sit in the house of his father or the house of his mother and see if he would be given a gift?" This money is unlawful, and he must estimate its value and give it as charity to good causes. If he is poor, then there is no blame upon him, and repentance, regret, and the resolve not to return to it are sufficient for him.