He is a monster. He has time on his side, reminds me of Lewa but Lewa is of course much more well rounded and overall the better player for now. But if he keeps scoring and progressing than he might become one of the best of his generation. He is/was a Madrista, but I would welcome him if he would change his mind.
Professional footballers aren't that attached to a club. Haaland would have absolutely no problem playing for Barca.
I'd imagine likes of Koke, Ramos, Pique straight up refusing to play for Madrid/Barca, but those cases are incredibly rare.