"I would have liked Niclas to stay for the final season of his contract, but having said I wasn't going to offer him another three-year deal I couldn't then turn round and say I wasn't prepared to let him go," said Keegan.
"He is not a greedy boy, but he was coming into the last year and I wasn't prepared to offer him the three years that he wanted.
"I promised his agent that if the right offer came in I wouldn't stand in his way and he has been given a fantastic opportunity by Borussia Dortmund and we got a very good price."