Thus, fair use need not even be raised as a defense unless the plaintiff first shows (or the defendant concedes) a prima facie case of copyright infringement.

and

However, binding agreements such as contracts or licence[sic] agreements may take precedence over fair use rights.

You can enter into a contract. There is no requirement that the original work is unpaid. Quite the opposite. If the original IP owner purchases it, since they own the IP there's no case of copyright infringement. And nothing stops the work creator from entering into contracts.

Edit: I'm not arguing this. This is literally the ONLY way to get paid for fair use materials. Period. You're wrong. Fair use is an anti-copyright lawsuit claim. There is no issue of copyright if you sell to the original copyright owners.

And even then... EVEN IF you were correct. Your claim that they cannot sell it to the original IP holders doesn't just allow them to steal the work. That's still theft.