What is the ruling on an employee taking money from entities he deals with, with the approval of his direct manager but without the knowledge of upper management, which has prohibited that?

The questioner is a private employee of this institution, and it is not permissible for him to take these funds except with the permission of its management, due to the saying of the Messenger of Allah, peace and blessings be upon him: "Gifts to workers are ghulūl (treachery/ill-gotten gains)." This is also based on his story with Ibn al-Lutbiyyah, who said: "This is for you, and this was gifted to me." The Prophet, peace and blessings be upon him, responded to him: "Why did he not sit in the house of his father or the house of his mother and see if anything would be gifted to him or not?!" His direct superior does not have the right to grant permission while the management forbids it, because he himself needs their permission.