Füllkrug played very well in Dortmund last year in my opinion. Dortmund made a good profit by selling him for a good transfer fee. Now they bought young Beier instead and will sell him for a higher price. If he shines well for a year or two, Beier can earn Dortmund big money. Guirassy will give the impression of being the other goal scorer.
Dortmund have always been able to do business like that every season so it is not surprising that they continue to do things like that to get more money. Because in general Dortmund have always been able to turn ordinary players into expensive players by honing and training them well before selling them for a fairly high price. But the focus at the moment is actually about Dortmund who could be a threat to other teams in terms of winning the Bundesliga trophy because Dortmund already has Guirassy as a player who will score goals for the team's victory.
Dortmund is not a team that has strong finances like Bayern Munich, so they need a large income every season to maintain the stability of the team so far and selling players is certainly an effective solution for them, what Dortmund doing not diference from majority of teams who have limited finances, so it's not surprising that they are financially stable but have difficulty in winning the championship trophy so far, Some time ago, Dortmund again sold the Fulkrug to another team and without a long time they were able to get another talented player, guirassy of course Of course, we can't deny that he is a great striker and Dortmund managed to get him at a very cheap price at the moment, while Beier, who is a talented young player and has quite a high goal scoring instinct of course just by honing his skills a little, it is not impossible if in the next few seasons Beier will can be sold at very high prices by dortmund.
What Dortmund is doing is good business at the moment, on the one hand they can continue to make money by selling star players and on the other hand they can still stabilize the club by continuing to bring in talented players all this time.
The whole team in Dortmund is based on selling players to other teams. As you already said, they are not a team with strong financial power like Bayern Munich. Dortmund's first priority is not even winning the Bundesliga title but they want to sell players and earn money and that's how everything works in Dortmund.
The interesting thing about Dortmund is they even have more fans to watch games in the stadium which can show they have much more potential than other teams in Germany.