It is with great regret that I have to resign my internship. I have already informed XYZ Company.
I knew when I applied that the position was unpaid, but I was under the impression that my time commitment during the summer would be limited to 20 hours a week, leaving me free to pursue paid employment while not serving as intern. Unfortunately, my supervisor at XYZ said that no other work commitments are allowed. I cannot afford this.
I understand that I won’t be earning any course credits for this internship. I am already scrambling to find another placement and may need to prevail upon you for another letter of recommendation.
I am sorry for any inconvenience this may cause you and the department. I am very disappointed myself, as I was looking forward to interning at XYZ Company.