Is it permissible to use money sent for tuition fees, which has already been paid from another source, to purchase other necessities that aid in studying, or must it be returned to the sender?

Your action regarding financial aid depends on the donor's intention. If it is an absolute gift that you can dispose of as you wish, then there is no blame upon you for that. However, if it is designated only for study expenses, then it must be used for that purpose and it is not permissible to spend it on anything else. You must return it to him if you violate his condition.