Is it permissible for a student to dispose of the fees for extra credit hours that were paid to the university but not registered, knowing that the remaining amount will be spent on university requirements?

You must refer to the one responsible for the tuition fees and inform him that you only registered for nine hours instead of fifteen. He has the option to permit you to benefit from the excess money for your studies or other needs, or to demand its return, and that is binding. It is not permissible for you to dispose of the excess without his permission, as the Messenger of Allah, peace and blessings be upon him, said: "The property of a Muslim is not lawful except with his willing consent."