I have continuously very much doubted this assumed 'release clause', official Or gentleman's implied.
But the recent narrative from Villa makes me think there might be something in it maybe.
He has 4 years on his contract. If he had no agreement to leave, it would simply be a case of 'we are not selling, he us our player on a long deal'. Even a 'it would take a crazy offer' maybe. The fact the reported stance seems to be they will tempt him with a new contract, rather than they won't let him go, might just suggest simply keeping him isn't that simple.
Probably not enough to convince me this release agreement exists, but does make me wonder a little bit.