Is the additional amount paid by the the company ($600) halal or haram?

It is not permissible for a worker or employee to accept a gift, because it is a form of prohibited bribery, and due to the corruption of integrity and swaying of hearts that results from it. The Prophet, peace and blessings be upon him, said to a man who was an agent for zakat collection and had been given a gift: "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?" Therefore, whatever is gifted to you in your capacity as an employee is not permissible for you to take, and must be returned.