Tianjin Quanjian are threatening to take legal action against Anthony Modeste and Cologne after the German club announced they had re-signed the striker.
Cologne released an official statement on Saturday to confirm Modeste, who had been training with their under-21 team, had rejoined on a contract spanning until 2023.
The 30-year-old initially left in July 2017 for a reported fee of €35 million (£31m/$40m), signing a deal scheduled to run until the end of 2020.
However, Modeste has been in a contract dispute with Tianjin since August and while his appeal for an early termination is yet to be resolved by FIFA, has now agreed a return to one of his former employers.
The Chinese Super League side responded to the news by issuing a statement, reminding both Modeste and Cologne that he is still registered as a Tianjin player while revealing they are prepared to take the matter to the Court of Arbitration for Sport if necessary.
"Modeste joined Tianjin Quanjian in 2017 and signed a contract valid until the end of 2020. Since August 2018, Modeste has left the team while remaining a registered player and he appealed to FIFA for an early termination of the contract," the statement read.
"Tianjin Quanjian has actively responded to the lawsuit, relying on the facts to reasonably refute the unreasonable demands of the player.
"Before FIFA made a decision, the club regretted the misjudgement of Cologne in signing Modeste and reminded Cologne that it might be responsible for breach of contract with the player.
"Tianjin Quanjian firmly believes that FIFA will make a fair decision on this case, and the club will take all measures to safeguard our legitimate rights and interests. Also, the club has made further preparations, to file a legal proceeding against Modeste and Cologne at the Court of Arbitration for Sport."
Modeste scored 45 goals in his first two-year spell with Cologne, who were relegated from the Bundesliga at the end of last season.
"Cologne has been very concerned about me and helped me tremendously in the past weeks after my return from China," he said in a statement on the club's website.
"I really want to repay this support and trust and do my part to make them successful. I cannot wait to get up for Cologne again."