Contract theory does not exist in situations involving criminal activity.

Let's think about this for a second.

If you make someone sign a contract stating that they are not a member of the law enforcement community so that you can sell them illegal substances, do you really think that would hold up in court?

You still sold an illegal substance, and there was still a witness that is more credible than you are.

Even if CT held up in this instance, the best you would get would be suing the cop for falsifying information on a legal document.

This wouldn't happen though, because legal documents cannot concern illegal actions.