The German, who had been on-loan in Turkey for two seasons, explained his reasons in an online post.
He wrote: "Hi everyone, today I terminated my contract with BESIKTAS.
"It's a shame it comes to an end like this but you should know that I have tried everything to solve this situation without any problems.
"I was very patient for months telling the board over and over again. Same things happened already last year. Unfortunately they haven't tried to solve this situational problem and even refused my suggestion to help by taking a pay cut.
"It's important to me that you know I really enjoyed playing for this club a lot. BESIKTAS can be proud having such passionate fans behind them always giving amazing support.
"You always supported me in good and bad times and I will always remember you in the best way!
"Also I want to say thank you to all my teammates, coaching staff including all people working for the club. You welcomed me with arms wide open from day one.
"Thank you so much! Champion Besiktas".