I think there is some serious Stockholm Syndrome going on between Dortmund and Bayern. Why would they take Mats back?? Götszche too. It's like verifying that going to Bayern to collect trophies is fine, you can always come back if you become too shit to play for them. It is so weird to me.
If they really had some balls they would save his big fee and massive wages and overpay for the next German CB-talent to get him to join them instead of Bayern, imo. That would be a better path to a future where they compete with Bayern, than this one, in my opinion at least.