It was billed as the dream transfer of one of Europe’s most promising players, yet the details surrounding Ousmane Dembélé’s transfer from French side Rennes to Borussia Dortmund last May has shone some intriguing light on how a multi-million euro industry functions behind the scenes. Featuring warring representatives and a club who have denied exerting pressure on a teenager to turn his back on his long-time advisers, the case has already come before the French Football Federation but remains unsettled, with Dembélé now likely to be called to give evidence in court over a dispute that could be worth up to €3m (£2.6m) in damages.

Featured in Tuesday’s edition of L’Équipe, the story has also been tracked by the Guardian over the past few months, although it began in 2011, when Badou Sambagué, a former Mali international who grew up in the same block of flats as Dembélé in the Normandy town of Évreux before becoming a lawyer, first took Dembélé to train with the Ligue 1 club Rennes.

Having impressed in the youth ranks, the skilful teenager was hoping to win a call-up to the first team by manager Philippe Montanier, who later joined Nottingham Forest, but found his opportunities limited. Dembélé was then offered the chance to join his boyhood friend Dayot Upamecano at the Austrian side Red Bull Salzburg, while also receiving interest from Manchester City among others. Thus, Sambagué and his associate, registered French agent Martial Kodjia, asked if Dembélé could break his youth-team contract at Rennes only to have their request angrily refused by the club. After much negotiation, Dembélé eventually agreed to accept his first professional contract in September 2015 but only if it included a release clause of €5m. According to L’Équipe, rather than at the club’s headquarters, the deal was signed in a hotel room at the Hotel Balthazar in Rennes because, in Sambagué’s view, he was no longer welcome at the club.

It was against this background that the then 18-year-old embarked on his first season as a professional. In November that year he scored his first senior goal in a 2-2 draw against Bordeaux and followed it up with another just before Christmas in a victory over Guingamp. His performances did not go unnoticed, with Barcelona, Manchester United, Liverpool, Bayern Munich and Arsenal among the clubs to make contact with Sambagué over Christmas. Borussia Dortmund also made their interest known and opened talks with the French club. “It was at this point that Rennes did everything they could to create a war,” he told the Guardian this week.

Fearing that they could lose their prized asset for just a fraction of his value, Rennes attempted to act. Through Moussa Sissoko, a French agent registered in Mali who also represented two of their other young stars, Sambagué alleges they sought to persuade Dembélé to sign a new contract not containing a release clause and to terminate the relationship between him and Kodjia.

“On 24 February 2016,” wrote L’Équipe, “he sent the following message to relatives: ‘If I do not go through Moussa Sissoko and Marco Lichtsteiner, they will stop the transfer.’” The brother of Juventus defender Stephan, Lichtsteiner is known as an influential agent in Germany who regularly collaborates with Sissoko.

Two days later, Kodjia was sent a letter signed by Dembélé which cancelled the agreement from September. According to Sambagué, this was a result of the undue pressure that had been placed upon the teenager. “Ousmane was very afraid. I tried to convince him not to give in and to stay strong,” he said. Sissoko told the Guardian that the allegations were “completely false”. “His old agent is telling stories,” he said. “I think that if Ousmane left his historical advisers, it was because he thought they were not good enough to manage his career,” Sissoko added to L’Équipe. “Each has its limits, and one must know how to recognise them.”

L’Équipe also alleges that Rennes had offered to pay Dembélé €5m – the value of his release clause – if he agreed to sign a new contract with the club and even to represent him in his future career, a practice that would be illegal under current Fifa rules. Rennes have denied this was the case and in a statement to the Guardian said it was a matter for the two agents in dispute. “The club has no direct involvement in this matter and wishes to remain neutral and impartial,” said the statement.

Despite apparently severing ties with Sambagué and Kodjia, Dembélé stayed in touch with his former advisers and met with Jürgen Klopp, the Liverpool manager, at the end of March. Manchester United also showed an interest in taking the player who had become the youngest in the history of Ligue 1 to reach 10 goals in a season – quicker than both Thierry Henry and Anthony Martial - although by that time his future destination already seemed determined. In June 2007, the Rennes’ owner, François-Henri Pinault, purchased a controlling stake in German sportswear manufacturer Puma, who also happen to be one of Dortmund’s main sponsors having bought shares in the club in 2014. There is no suggestion that Pinault’s influence resulted in Dembélé moving to the German side, but Sambagué believes that Rennes had “closed the door to all of the clubs except Dortmund, who were willing to pay more than the clause”.

Having seen an offer of €8m rejected, Dortmund eventually agreed a €15m fee. But there was to be one more hitch. When he arrived to formalise his transfer, Dembélé was accompanied by Kodjia after signing a new mandate with him the night before, which was designed to reward his long-term advisers for their assistance. Initially, Dortmund refused to go through with the deal and it was only once Dembélé had formalised his five-year contract with the club that Kodjia realised his name had been omitted from the paperwork.

On his return from Germany, Kodjia and Sambagué contacted the FFF’s Commission Fédérale des Agents to register a complaint against Sissoko and Rennes. They are seeking €3m in damages – the commission banked by Sissoko from the deal – and were offered almost half of that in November as a settlement. A further meeting is scheduled for next month, with Kodjia and Sambagué also considering suing Dembélé, who has refused to comment on the case, for breach of contract. The striker, now a full France international who has continued his meteoric rise on the pitch with four goals and seven assists for Dortmund this season, could also be called as the only witness in an attempt to unravel the truth in this very modern transfer tale.