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
we will have great games like:
FC Bayern Munich - RB Leipzig
this will be a great game and my bet was that Bayern Munich will win this game
from the history of head to head that occurred between leipzig vs bayern, leipzig only won once at Bayern's home. then this match is not a difficult thing for Bayern, they will definitely win