Dortmund is used to letting their best players go once they profit from the deal, which is why I think they are still suffering to win the Bundesliga ahead of Bayern Munich. It is only Marco Ruise that is patient enough to stay at the club despite different offers from different teams.
Haven’t no one advise them to stop doing that or they’re just after money in the club only. I remember Lewandowski was one their players they sold to Bayern Munich that season, can they ever imagine how good Lewandowski will become, a very great striker and the best in Bundesliga during his stay in Bayern. I hope they change their perspective and learn to keep players that are still valuable to the team.
But it is indeed a business for them too because if they don't do that then their condition will not be too good in terms of their financial stability.
Of course, losing this player will also have an impact on the stability of the club, but forcing a player to stay whose contract is about to expire is risky for Dortmund.
Now what they need to do is no problem their star players leave but not to the rival club is the most important.