I agree that the prospect of playing for us is exciting, but let's not forget:
-He's German
-He's still young and has plenty of time to make a big money move
-Dortmund signed him as a relative unheard of player. He's surely grateful to them for that and it could make him more reluctant to leave after only three seasons.
Not that I couldn't see him wanting to join, but I certainly don't think it's a formality.
Same could be said about Sane, look at how that turned out ;-)
I get what you're saying though