Hakan Calhanoglu has been banned from football for four months after being found to have breached his contract with Trabzonspor FC in 2011.
The Bayer Leverkusen midfielder had originally signed a contract with Trabzonspor when he was 17, agreeing to play for them from the 2012-13 season onwards.
The offence relates to when the attacking midfielder agreed to sign for the Turkish side before opting instead to extend his contract with German club Karlsruher SC.
Trabzonspor reportedly payed Calhanoglu and his agent a sum of €100,000 when they put pen to paper in 2011.
The Super Lig side filed a complaint with the FIFA Dispute Resolution Chamber in April 2013 and the body found Calhanoglu to have breached his contract there without just cause in January 2016, handing the player a four-month ban.
However, that sentence was temporarily suspended when Calhanoglu made his appeal to the Court of Arbitration for Sport (CAS), but the latter also ruled in favour of Trabzonspor on Thursday and ordered the Turkey international to pay €100,000 to the club.
"Hakan Calhanoglu is ordered to pay €100,000 to Trabzonspor FC," a statement from the CAS read.
“Furthermore, the four-month ban imposed on Hakan Calhanoglu by the FIFA Dispute Resolution Chamber, which was suspended at the beginning of the CAS proceedings, is reinstated with immediate effect.”
Leverkusen remain in Champions League contention – they face Atletico Madrid in the last 16 later this month – and sit ninth in the Bundesliga, seven points adrift of the top four.
The club have reacted with fury at being deprived of one of their star players for an incident that took place long before they signed him from Hamburg in 2014.
Hakan Calhanoglu has received a 4-month suspension from FIFA
We're standing by you, @hakanc10 !
More details coming shortly... pic.twitter.com/tlzcHKzCVa — Bayer 04 Leverkusen (@bayer04_en) February 2, 2017
"Of course, we regret this decision which is completely incomprehensible for us," said sporting director Rudi Voller.
"It's a heavy blow for Hakan but also for us. We are also being heavily punished, although Bayer Leverkusen had nothing to do with the events in 2011. We are now missing a very important player in a crucial phase of the season."
Michael Schade, Leverkusen's CEO, added: "In addition to the player, it also affects the club that played absolutely no part in the events of the time. Unfortunately we are left with no alternative but to accept the decision."
If Calhanoglu - who has netted six goals in 15 Bundesliga appearances this term - is indeed to sit out, it will be a blow to the club's season and their hopes for qualifying for next season's Europa League.