The 21-year-old's deal with Monchengladbach still has 18 months left to run, but he has turned down the offer of an extension.
He said: "I think everyone knows that I'm Borussia through and through, so it was a hard decision for me. I hope the fans will understand and accept it.
"I owe the club an incredible amount. We want to play together as a team for a good second half of the season now."