As someone pointed out earlier in the thread, we did exactly the same with De Gea. All summer we said he's not for sale, we weren't interested in any offers etc, yet Real Madrid put in a sensible offer and we accepted, and if not for a dodgy fax machine, we'd have lost De Gea that summer.
If the right offer comes in, then the player goes. ALL players have a price, regardless of how important they are!
As i say, Dortmund will be saying one thing to the media and another thing to United. They have to maintain face in all of this. They've said he's not for sale, if he goes, they have to paint it as it was HIS decision to go, not theirs. They can spin a sickness to say he refused to play due to being ill, and if he goes, theres the excuse to paint him as the bad guy. If he stays, then theres no proof that he wasn't genuinely ill.