If Haaland wanted to remain in Dortmund the club will not sell him to Manchester City, i know he was contracted to them, but the boy had already explained his position to the club, telling them that he wants to go, it will go against the club's interest to hold down a player that wants to go, Dortmund could do that and have a player that is unhappy, who doesn't show any enthusiasm in the locker room, it might even affect the player's form, and Dortmund will gain nothing if that happens, they might even end up selling the player cheaper than they may have sold him before. I like the club's belief, they will keep just the players that want to stay.
If Dortmund keep Haaland until his contract expires it is certain that he cannot perform with such dedication, maybe his performance will decrease because he is not fully with Dortmund. But this has never happened at Dortmund, reflecting on last season's Jadon Sancho was also sold to Manchester United while still under contract with the next few seasons.
Dortmund is a typical club that prioritizes profits and business, if players are demanded by other clubs at a high enough price, they do not hesitate to increase the duration of the contract for their players, immediately take the initiative to sell and look for other alternatives and can earn big profits in the future.