I think you should take the money, try to retain as much of my copyrights as possible (the law may help you do this implicitly rather than explicitly, which may be helpful), and refrain from doing anything else until after the student misuses your work.
Why? Consider that:
- Plagiarism is not a crime.
- Copyright violation is a crime.
- Neither has occurred yet.
- You have done the work in good faith.
- You are not responsible for preventing others from plagiarizing your work.
Refunding him and keeping the work would be unfair to you because you have already done the work. You have every right to keep the money for the work you have done.
Refunding him and keeping the work would be unfair to him too, since he has every right to obtain the work from you that he has paid you for and waited for. Even if you refund his money, you can't refund the time that he has spent waiting for you to do the work, so you would arguably be liable for damages too.
If your work is actually plagiarized, then you will have a basis for complaining academically and/or legally, and it will be more difficult for your client to view you as a "snitch", etc. if he violates your copyright.
If you let his institution know before the deed occurs, then I'd argue you're doing something morally wrong: you're putting him in trouble for an act that he has not even committed yet. (This might be fine if the act involved danger, but it doesn't.)