This looks so random. He's had a couple of games in the third division and one half of first team football, where he made quite a prominent mistake for Stuttgart's "famous" last minute 3:3. And now he's worth €15m?
I can't judge how spectacular his second team performances were, but you'd think if he was that special Dortmund would incorporate him into their first team themselves, especially since they actually need some replenishment at CB, with Hummels being in the last year(s) of his career.
Is this some Kevin Schade type of situation?