Manchester City officials say the club didn't break the rules when it signed French teenager Jeremy Helan in 2008.
Helan, 17, used to play for French side Rennes, but left them to join City. Now Rennes have complained to Fifa.
Helan is the latest in a series of transfers of young French players to make the news, after Chelsea's 18-month transfer ban for signing Gael Kakuta.
Another French team has said it's not happy with Manchester United over the signing of 16-year-old Paul Pogba.
Le Havre claim that United offered Pogba's family lots of money if they moved to England and Pogba signed for United.
The Premier League champions say they didn't break any rules, and have threatened to sue Le Havre.
City have said that Helan and Rennes were arguing about the player's contract before they signed him.
The club added that it's confident that it didn't try to persuade Helan to break his contract with the French team.
The situation is complicated because Rennes say Helan agreed to sign for them when he was 13 years old.
European law says a contract can't be signed until a player is older than 16. Some countries operate under different laws as well.