Is it permissible to take the remainder of the amount given by the uncle as assistance to pay university fees, knowing that he requested the cost of six courses but decided to study only five?

You are not permitted to benefit from the excess money beyond what was specified for you. You can inform your uncle of this and return it to him. If he then gives it to you, there is no sin upon you in benefiting from it. If the donor specifies that the gift be spent on a particular item, then it must be spent on that item, in consideration of his purpose. Otherwise, the recipient owns it and may dispose of it as he wishes.