If Haaland doesn't sell this year and Dortmund doesn't extend his contract, his contract expires next year. BvB Dortmund could lose a football star like Haaland for free with no revenue. They are also aware of this. So if the contract cannot be renewed, it would be more profitable for them to sell it.
Haaland contract expires in June 2024 but he can quit Dortmund next year after paying €75m.
I read Chelsea offered around €150m to Dortmund, accept it or leave it, I think it is a golden chance for Dortmund otherwise they will lose him next year most probably for half that number!
At the moment Dortmund should sell Haaland. 150m is a very large amount of money. Maybe Haaland should be sold, otherwise they can add some experienced players to the team by spending more money. Then they can expect to win the title. Relying on Haaland alone is not possible to win the title.