The owner of the French club must repay money by October this year before they will be allowed to sign players again
Lens finished second in Ligue 2 last season but their promotion to the top flight was in doubt for a period as they delayed providing guarantees that they had they were financially stable.
Despite getting their promotion guaranteed, they have since been told they cannot sign any new players by the financial watchdog of the French game (DNCG) until Mammadov injects more money in to the club.
A statement on Lens' website read: "The DNCG has temporarily prohibited Racing Club Lens from recruiting players, pending the fulfilment of the commitment of the shareholder to prepay amounts due in October 2014."
In reaction to this revelation, Sochaux have launched a protest, as they feel that they should still be able to take the place of the Nord club in the top flight, despite the fact that they kicked off the Ligue 2 season last Saturday with a 1-0 home defeat against an Orleans side just promoted from the third flight.
"Following the FFF's decision to reinstate Lens in Ligue 1 on July 25, Sochaux said we would remain vigilant on the principle of equal treatment of clubs by the bodies of French football," they explained in a statement on their official website.
"Remember that one of the tasks and duties of the French FA is to ensure the sustainability of sports clubs to ensure sporting fairness. We see that in this case fundamental fairness has not been respected by Lens."
Although the DNGC's decisions court controversy frequently, 2014 has proven to be a particularly difficult summer for French football's FFP regulator. Their decision to prevent Luzenac taking their place in Ligue 2 being overturned by the legal system, leaving the second flight with 21 teams - a situation that has still not been adequately resolved ahead of the scheduled second round of fixtures on Friday.