Being paid for helping my friend in his graded assignment and asking him to learn afterwards. Haram?

I have Helped my friend in one of his graded assignments and he paid me. Now I offered him to teach him that topic for free, and he learns too. Now here the student have learnt the topic and is capable to does the task in the future. The grading is also done in an absolute system unlike relative system (On curves) so no body is harmed. Plus if his degree states that he is capable to doing that task, then that's also true. In this case no body is hurt. I'm I allowed to keep the cash?