At this point I don't even think money is the reason they sell players to Bayern Munich. I said so because they sell their players very cheap to Bayern. If any club that's not Bayern Munich goes for a Dortmund player, they will slap a very high price tag on the player, but if it was Bayern, they player will leave for almost nothing.
Just recently Raphaël Guerreiro joined Bayern Munich from Dortmund for free. So how exactly is Dortmund making any money out of that.
Some times I wonder if all the Bundesliga teams play for Bayern Munich, especially Dortmund.
Its not doubt about football business era right now by selling their player to other team with higher values, but what did by Dortmund seems contrast because sell their player to rival teams or give chance to Bayern keep dominance in Bundesliga by selling their star player. Recently, Dortmund failed give extend contract to Raphaël Guerreiro and he has joined Bayern Munich with free agent and seems most disappointed for Dortmund losing potential player with free.
Dortmund has made much money from transfer player, they have great scout talent how to make many young player have consistent performance and increasing up their values after one until three years with Dortmund. But one weakness side from Dortmund they not try to stop Bayern keep dominance in Bundesliga and has priority how to promote young player on every season for increasing their transfer market values.