Thats why Dortmund declined bids over 50m€ for Reus and Hummels over the years..thats why Dortmund just these days refused to sell Aubameyang for over 40m€ and declared him unsellable..Aubameyang
Dortmund just selling players with expiring contract or when they have a clause of course. It is a well known fact, that no player having a clause anymore.
No club can go to Dortmund and grab their players..many tried, all failed. (All players left had clauses or just 1 year contract).
Next year it doesnt look that great for them though, as Hummels' contract is running down and next year they must sell him, if he decides not to extend (and I think he wont extend and moving to EPL, after United tried for 3 consecutive seasons). Same story with Gündogan, as his 1year contract extension looks like a joke..just delayed.
Germans tend to be very stubborn when it comes to selling players and Bayern, Dortmund and Schalke all three r tough to deal with and having their principles..also Leverkusen and Gladbach..u cannot go there and demand something..a no is a no.