They will have to sell, full stop.
Dortmund have zero leverage. Haaland will tell them which club he is joining and that will be that.
No club, certainly not Dortmund, can afford to turn down a massive premium when the other choice is a massive hit.
As I have said before, perfect storm in so many ways.
They can still refuse to sell. Or claim to refuse to sell. They'd be silly to, but they still get 75m the following year, and any club will know they still need to pay 75m. So they can push the asking price to what they see.
It is like a player in the last year of his contract, except instead of getti g him for free a club gets him for 75m. The year before though, they can ask for more, including not selling.
obviously, they Will sell. But the dance has to happen first.