Goalkeeper Heinz Muller has left Barnsley and completed his £600,000 move to FSV Mainz.
The German confirmed earlier in June that he was set to return to his homeland and he has now ended his two-season spell at Oakwell.
Mainz will be playing in the Bundesliga next term, and Tykes boss Simon Davey did not want to deny the 31-year-old the opportunity to play at that level.
Davey told Barnsley'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."
Meanwhile, Barnsley have also signed striker Onome Sodje from York City and he will join the likes of Reuben Noble-Lazarus, Jamil Adam and Jordan Hibbert in a new development squad.