The 21-year-old, who is regarded as one of the best young goalkeepers in world football, has been widely linked with a move to Barcelona at the end of the season.
Talks have stalled in recent weeks, but the German has now claimed that a decision on his future will be revealed "in the next few days".
"I will make a decision in the next few days. Those responsible know what is going to happen," he told Bild.
Barcelona are expected to lose current number one Victor Valdes when his deal expires at the end of the season.