Can't sell Robben, can't buy German kids (Schalke wonder boys) for less than 50 Mill...
Why?
Sometimes teams do not wish to sell their players to other teams of the same league unless the players submit a transfer request or their contract are about to expire.
Schalke are not desperately short of cash and do not need to sell. Therefore, you could try praising either their manager, or the players themselves in a bid to unsettle them and result in them submitting transfer requests or maybe doing both.
As for Robben, he's already 29 years old and also commands a high wage which is probably why no one's interested in him. If you're really desperate to sell, you could try offering way below his value or maybe even for free. Offer him on loan if all fails as it at least gives you a chance to free up wages.