What is the ruling of Islamic law concerning taking excess money by mistake from an employee, and a friend of mine seizing it and refusing to return it, knowing that I was the one who initially received the money and that I informed him of his responsibility?

The guarantor of the money is the person who took it from the ticket agent, and he must return it to the entity from which it was taken. Then, he can claim reimbursement from the other person for what he paid, based on the Prophet's (peace be upon him) saying: "Upon the hand is what it has taken until it returns it." Among the causes of guarantee are: the hand (possession), causing loss, and causing damage. If the taker knows that the money is not rightfully his, he must repent and feel remorse.