I don't object to a release clause in principle for a footballer like Haaland. It would have to be a robust fee, but if the player turns out to be happy at OT he can always extend his contract...and if he's not happy and wants to leave, he's going to leave anyway.
Given our current striker situation...
Ronaldo, brilliant but aging quickly
Cavani, brilliant but too often unfit, is aging and is out no matter what at the end of the season
Greenwood, brilliant but still very young
Rashford, crumbling before our eyes and in any event is not a striker
Martial, a player who can't be bothered with working hard
...it would in no way be insane to bring in Haaland if there were a way to make it work.
We let Cavani and Martial go. That's the easy part. Ronaldo is a tricky one but it's not hard to envision Ronaldo making it work. Greenwood still has a long career ahead of him but he needs to learn how to work harder and smarter on the pitch. Rashford can come on when needed in support of Haaland. Four strikers: Haaland, Ronaldo, Greenwood and Rashford. Not bad at all. Ridiculously expensive so I'm not sure how realistic this is though. But we should put up a fight as it would be horrifying to see him go to City or Chelsea, or even worse -- Liverpool.