But that's not true. Transfer money is one instant push, the club has to pay immense taxes on it and can't invest it in full. There's also the problem of wages, signing several players or trying to sign one new star player for the incoming money might not be covered by it. There's a good chance that the club makes a financial loss and is weaker than before after all the transfers are done, just look at Liverpool after Suarez left or Tottenham after Bale left.
Dortmund also don't have an owner who wants to make money, they're still a football club, (mostly) fan owned, and want to play with the best team possible. Without the player himself pushing out, there's no way they sell. It simply doesn't happen, never happened in Germany. You can't give me a single example where that happened and the Beckham transfer is in no way comparable.
Convince Reus to leave and offer a shitload of money to Dortmund? They'll consider it, there will be a price, even though with the contract situation at the moment, it'll be way above what he's worth. If Reus doesn't want to leave? No way, will Dortmund push him out the door, it makes no sense at all, certainly not longterm in a financial sense. The only way for Dortmund to grow further as a club is to be successful on the pitch longterm and keeping Reus is right now the most important piece for that, because they can't attract or pay a comparably great player.