Hannover has sold forward
The 25-year-old requested a transfer after the Lions made their approach and the Bundesliga club accepted the offer for the former Rot Weiss Oberhausen forward, who moved to Lower Saxony in 2010.
"We accepted the player's wishes by releasing him early," said Hannover's director of sport
Stoppelkamp made 44 Bundesliga appearances for the Reds, playing an additional five games in the UEFA Europa League last season.
Duisburg's relegation to the third division of German football has been confirmed after its appeal against the German Football League's (DFL) refusal to award a licence was rejected.