For sure they making money and made a lot in the last years with selling players , but mostly they have to sell them because of there higher salary they want.
And on top of that they want trophies but they got unlucky and other teams was better in that seasons.
If they sign the players at a tender age and when they are not in hot form there is no way the players will be demanding high salaries when their contract is still running in the team, I understand that Dortmund wants to keep making money since they can not win trophies with the players they have but I think they can still have good players after selling some all they should do is to spend money to buy young players then groom some to sell and some to fight for the trophy.