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.
I think it's not only dortmund who have this habit, other big teams like madrid or barca will also release their players if they make big profits for them
I don't think Madrid is included there because the average Madrid player who is sold is because they are no longer used there and have been left out by other players who are shining.
For example, when we saw the Galacticos era, especially the Cristiano era, Kaka and Ozil entered as retainers of Madrid, many players were wasted there which incidentally even though these players were still very worthy, just like Robin, Sneijder and even Raul.
On the other hand, for Dortmund itself, it seems that their priority is not really chasing the champions because their current rival is quite strong and Bayern's hegemony is still continuing so it would be more profitable to sell their players, which are quite expensive.