Yes he has for sure some awesome stats for the last season and years for his age of 21.
I guess he wanted to make the next step to grow and also wanted to play on a higher Level by moving to Dortmund even there are a lot of other good players.
I don't remember exactly when and what age Haaland went to Man City. But Haaland at Man City received tremendous support so he was able to perform well. Man City's players have good control that helps Haaland perform well. Meanwhile, Beier at Dortmund is indeed a good step and it is an improvement. I don't know for sure if Dortmund is doing business or if they are indeed aiming for the championship next season. But indeed Dortmund is always good at bringing in young players then they are made better and sell more expensive.
Haaland was 21-22 when he got there, so considering he is 24 years old and it was two full seasons with City now, we can say he was 22. The difference is that Beier is already 21, so he can't be going to City level team in a single season.
But we can see at around 24, after three full seasons with Dortmund, if he is awesome and playing very well, he could be bought by some richer and strong team without a doubt. That is what Dortmund is aiming too, they are aiming to sell him for 100+ million in 2-3 seasons at best, which is not something they haven't done before, this team keeps finding talented players all around the world, pay 10-30 levels and then sell them for a lot more.
I think, Beier will adapt quickly and score more than last season, and we won't wait 2-3 years to sell him for a big fee. And i expect Dortmund can profit from Beier next year. If he performs well this season and scores more than last year. And we'll see some big clubs approach him.
Even though it won't be City's level. But, it's fine if we get a big offer for Beier after we paid his former club 30 million for him. And If we'll able to sell him for 60/70 mio, and that's good.