Bayer Leverkusen have confirmed their midfielder Hakan Calhanoglu is involved in a legal dispute with FIFA that relates to a contract he signed with Trabzonspor in 2011.
The Bundesliga club expects the proceedings to last for "several months", during which time Calhanoglu will remain eligible to play.
The 22-year-old German-born Turkey international came through the ranks at Karlsruher, who he represented between 2009 and 2013 - albeit on-loan from Hamburg during the 2012-13 season.
As a 17-year-old, Calhanoglu signed a deal with Trabzonspor to join them for 2012-13 but the move collapsed.
He joined Leverkusen from Hamburg on a five-year deal in 2014 and his current employers issued a statement to clarify that the dispute does not relate to this transfer.
"FIFA and Bayer 04 Leverkusen player Hakan Calhanoglu are currently involved in a legal dispute," the statement read.
"The case relates to a contract with the Turkish first division club Trabzonspor who Calhanoglu signed for as a minor at the age of 17. The agreement envisaged the midfielder playing for Trabzonspor from the 2012-2013 campaign.
"The move did not take place at the time primarily due to the lack of agreement between Hakan's former club Karlsruher SC and Trabzonspor.
"The case is expected to last for several months. Hakan Calhanoglu remains entitled to continue playing football."
Calhanoglu has scored eight goals in 39 matches this season for Leverkusen, who will aim to turn around a 2-0 deficit in the home leg of their Europa League last -16 clash against Villarreal on Thursday.
Karlsruher later released a statement of their own, with president Ingo Wellenreuther denying any negotiations took place between the 2.Bundesliga club and Trabzonspor over Calhanoglu.
"FIFA has stated that the contracts of the Karlsruher SC and Hakan Calhanoglu were all legal." Wellenreuther said.
"This is the case for his youth contract between 2009 and 2012, his professional contract from March 2012 and the transfer/loan contract with Hamburg from August 2012.
"There were no negotiations between Karlsruher and Trabzonspor for a transfer because at no time was there an appraoch from Trabzonspor. At the time when the [March 2012] contract was signed Karlsruhe didn't know anything about any contracts or payments of Trabzonspor."
After some media reports in September 2012, that the player have had contact with Trabzonspor in 2011 Karlsruhe asked Trabzon for a written report on this case but there were no reaction at all until 2014 when a filing of action was made. This lawsuit on KSC was rejected completely by FIFA.