I try to imagine if the situation was reversed, how we would feel. We have a German international who only moved a couple seasons ago, and a big German club is in for him after a great year. He's done really well for us, but we know he's not a United fan or really has any ties to us. We're not about to win the league anytime soon, and haven't really come close in the CL.
I would expect that player to leave, and more or less with my blessing as a fan. I would want us to hold out so we don't get fleeced, but wouldn't care about 10m one way or the other. But, I would have wanted us to get him out early, so if we had to replace him that could also happen early.
Because at the back of my head, the difference between elite and the rest is a fraction of a percentage. And if his motivation is slightly down, that will show on the pitch.
So we'll see. I don't think if you're the Dortmund coaching staff you can completely trust that he - a player with known attitude issues in the past - is going to be as motivated as last year. And 100m is a huge fee for such a relatively unproven player. With that and their scouting network they could easily pick out 3 or 4 more talents to improve them in other areas. Or keep 50m in case of a winter spike in Covid.