Is it permissible for a student to take the discounted amount from tuition fees without the knowledge of his father, who sends him the full expenses, on the pretext that it is the fruit of his diligence in studying?

What is incumbent upon you is to return the remaining amount to your father, or to inform him of the reality of the deduction so that he may feel at ease with you. You must reciprocate his kindness with honesty, for the fruit of your diligence is your mastery of learning and acting upon it.