"To explain why he's banned: while he was playing for Karlsruhe, he reached an agreement with Trabzonspor about a transfer to them. However, he then extended his contract at Karlsruhe and later transferred to Hamburg. Both CAS and FIFA rule this as a breach of contract, and thus he's now been banned for four months from all competitive matches.
While I understand why they came to this conclusion, banning him this long also severely punishes Leverkusen, who had nothing to do with any of this."
TL;DR, Signed a contract to move to Turkish club, but then signed an extension with Karlsruhe instead.