Haaland is a big part of Dortmund right now. In the absence of him due to an injury, we saw how much Dortmund had a hard time beating their opponents. I mean that they had scoring problems more. Because Haaland has scored most of their goals I guess. Considering this fact, I think that Dortmund would be much weaker than now for sure if they lose Haaland to some other club. I'm pretty sure that Dortmund would do their best to keep him here this summer also but it doesn't seem probable to me.
Dortmund isn't a team that try to keep players for a long time. Once they find another young talent they will sell Haaland to another club before his market value drops.
Now indeed Dortmund will have hard time playing games without him but this will eventually happen in the future so better to get used on not counting on only 1 player.