It cannot be denied, but that is what Dortmund always does, we can all see that there are many young players who are currently appearing impressive and are capable of becoming star players for the team they defend, and are also players who have been developed by Dortmund in the past.
No one cares about how much Dortmund used to spend money to bring in these players because the most important thing is that they can play very well and are able to have above average playing skills that can make big contribution to the team that has bought them.
Moreover, developing players to become greater and better is not easy, this is why Dortmund always puts high price on every player that other team want.

But speaking of Guraissy, who is new player for Dortmund, if he succeeds in becoming reliable goal machine in the first season, Dortmund should keep him for at least one more season until they actually get replacement.
They have lost their opportunity and also their popularity as the team that is at the top in the race for the title, Dortmund must regain it first before selling the players and making profit.