I think we should not forget where Borussia Dortmund is coming from. In the late 1990s and the early 2000s they were at the same level as Bayern Munich or maybe even ahead of them a few times. In order to do that they had to make very high debts though which almost led them into bankruptcy afterwards. Then Jürgen Klopp came and the club has risen again and they even made it into the Champions League final back in 2013. Still they are pretty far behind Bayern in terms of financials.
It's more than two decade ago. Lots of things are changing right now. That's why dortmund was using strategy to sell the player after developed it as a way to stabilizing the financial of club. Dortmund is not as rich as bayern. It must also think the expense by the club as well.
Bayern is always raiding dortmund's player which was also making the club to be even weaker when it comes to the new season. This strategy is a very bad strategy since dortmund was the only potential competitor in bundesliga for bayern.
Buying the good players from dortmund = winning the league for bayern. The investors must be going in. Bundesliga will always be like that forever without the hand from investors to build the clubs.
That's why dortmnd has no choice other than trying to use such strategy as a way to keep its financial stable.
I put a lot of respects for dortmund as its hard work to make sure the club will not face financial trouble that will be affecting its players.