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!