I think it's stupid that he wants to leave so badly. Can't he see that Bundesliga is on the rise? Premier League is only falling. Only if he leaves to Juve I can somewhat understand but even their League is in decline. But whatever, I'm sure that in a few years he will regret it. Sahin isn't much happier now than he was two years ago either...
And besdides, you shouldn't keep players that don't want to stay. If he thinks he's too good for BVB, then let him leave. He will find out sooner or later.
I agree Herrmann is a great addition to your squad. But indeed, not as a striker, more for the place where Kuba plays now.