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.
One of Borussia Dortmund's characteristics is that they train the players they buy well. They identify especially young and potential players and sell them to important teams in Europe after they have been trained well. There are many examples of this, but the latest example is Jude Bellingham. Beier currently has this potential. Since they transferred him at the age of 21, they can sell him to a big team again in a few years for a very high transfer fee. Borussia Dortmund does this job very well.