The 31-year-old returns home after spending two seasons at Oakwell, where he arrived in a £150,000 deal from Norwegian club Lillestrom.
Barnsley manager Simon Davey told the club's official website: "Heinz has been given the opportunity to play in the German top flight and it was the right deal for Barnsley Football Club.
"It is a good deal for both the club and for the player and we wish him well in the future."
Muller made a total of 69 first-team appearances for the Tykes and established himself as one of the outstanding goalkeepers in the Championship.
Meanwhile, Barnsley have also signed 20-year-old striker Onome Sodje from York and he will join the likes of Reuben Noble-Lazarus, Jamil Adam and Jordan Hibbert in the Tykes' new development squad.
Source: Team_Talk
