Lens have accused Chelsea of offering the family of Gael Kakuta 'a lot of money' in order to secure the youngster, and they are confident in their case.
Fifa announced on Thursday that the Blues have been banned from signing new players in the next two transfer windows following a complaint regarding the signing of Kakuta in 2007.
Ligue 1 outfit Lens claim that Chelsea convinced the midfielder to break a youth pre-contract which would have guaranteed the starlet signed professional terms when he reached the age of 16.
Didier Roudet, Lens' general secretary, is in no doubt that the Premier League side, who are planning to appeal against their punishment, offered Kakuta's family incentives in order to ignore the agreement.
"We sat with them (Chelsea) and told them the player was under contract but they argued that he was not," Roudet said in the Daily Mail.
"All I can say is that our lawyers are very confident of their position and so are Fifa.
"Lens are a little team. Chelsea are a big team with a lot of money and, for sure, the player's family were offered a lot of money. We could not keep the player."
Phillipe Piat co-president of the French players' union and president of FIFPro, the international players' union, said: "At 14, Kakuta and his father signed a youth pre-contract promising to sign for Lens when he turned 16.
"When he finished his youth development Lens wished to apply the contract and bring the player back but his father no longer agreed to this. He wanted to go to Chelsea."