Links to Web pages and tweets are not enough, as these can be erased, making it look as though permission was never given (and getting YouTube in trouble).
You need something more concrete, like a digitally-signed e-mail or something on paper.
The Creative Commons attribution license doesn't allow unlimited commercial use without conditions. It requires attribution in a manner specified by the copyright holder. Also, since the status of a video can be changed at any time on YouTube, CC licenses applied to a video by the uploader may not be acceptable in all situations (a real license cannot simply be rescinded unilaterally in the way that YouTube allows).
In summary, you need to come up with an agreement that is permanent (cannot be deleted with the click of a mouse) and clearly identifies the copyright holder, the licensed content, and the licensee in a way that rules out any possibility of someone else pretending to be him. Ideally, you should consult a lawyer about this. At the minimum, you'll need something other than a pointer to a Web page or a tweet—those are usually just too ephemeral to protect all parties concerned.