FIFA could investigate the transfer of Milan Badelj to Hamburg after a report from the German player agents' union.
Frank Arnesen, the club's director of football, faced criticism for using two agents, one of them reportedly unlicensed, in the completion of the deal that brought Badelj to Germany from Dinamo Zagreb. Concerns were also voiced over a possible increase in the transfer fee.
Earlier this month, Arnesen defended the fee - thought to have been around £3 million - after Dejan Joksimovic, one of Badelj's repreantatives, had questioned the use of Vlado Lemic, another agent, by Arnesen.
Joksimovic suggested that may have resulted in Hamburg paying more money than they needed to, telling SportBild: "The final deal was a very bad deal for Hamburg because they paid too much of a transfer fee and a second player agent. The contract of the player was about to expire. He just wanted to link up with Hamburg."
But Arnesen said: "There was an offer from Fenerbahce. They wanted to pay £3.6 million straight. We managed to grab the player cheaper in the end, about £350,000 cheaper. It's a success."
SportBild said FIFA could investigate after the DFVV union reported the case. DFVV chief executive Dr Gregor Reiter told Sky News: "Our main concern is to investigate if there have been any violations."
But Hamburg chief executive Carl Jarchow stated: "Everyone on the board agrees that the transfer fee and the conditions were reasonable."
The club's media director Jorn Wolf told Morgenpost: "We have not heard from FIFA yet."
In the worst case scenario for the Bundesliga strugglers, a FIFA investigation could theoretically lead to a points deduction if violations are found.