Gonçalo Reis has spent five months stewing on the betrayal he felt when he was sacked as the agent to Tiago Manuel Dias Correia – Bébé – who then, two days later, moved to Manchester United for €9m, the most improbable signing in the career of Sir Alex Ferguson, who admitted he had never seen the player kick a ball.

A 20-year-old who spent his youth in care near Lisbon after being abandoned by his parents, Bébé had played only one season, 26 matches, for Estrela da Amadora in the Portuguese third division, before Reis negotiated a move for him to a first division club, Vitória Guimarães. Last summer Bébé played four pre-season friendlies for Vitória, and sacked Reis as his agent on 5 August by letter, which Reis says he received on 9 August; then Bébé was the subject of a deal with United that works out as £7.5m on 11 August.

United confirmed subsequently that Jorge Mendes, agent to José Mourinho, who has become a huge influence in Portuguese football and previously negotiated the moves by Cristiano Ronaldo, Nani and Anderson to Old Trafford, acted for Bébé. Vitória's directors stated at their annual meeting late last year that Mendes, after seemingly less than a week's work, had been paid €3.6m of the €9m fee. Reis had a two-year contract to act as Bébé's agent from August 2009 to August this year.

Now Reis says he is "seriously considering" making a formal complaint to Fifa about the manner in which he lost his client to Mendes. "I spotted Bébé playing," he recalls, "I could see he had talent but, because of the person he is and his very poor and difficult background, I had to look after him very closely as a person, as well as be his agent. I negotiated his move first to Estrela, then to Vitória, then last summer he went missing on me. I read that he had agreed an improved contract with a €9m buyout clause at Vitória, then I suddenly received this letter in which he purported to say our relationship was at an end. Two days later he went to Manchester United for €9m. I am now very seriously considering making a complaint to Fifa."

Mendes has made no comment on his role in taking Bébé to United, or to confirm whether he did indeed receive €3.6m of the €9m fee, as stated by Vitória's directors. Reports in Portugal of the club's annual meeting last October said Mendes made the bulk of his money by selling the player's "economic rights" to United, rather than for acting as his agent. The contract Reis says he was negotiating for Bébé at Vitória was that the player himself would keep 30% of his economic rights, meaning he would receive that proportion of any transfer fee Vitória sold him for. Vitória's directors reported that Bébé did indeed have such a clause, but that Mendes, before agreeing the deal with United, had bought Bébé's 30% of his own economic rights – for €100,000. When Bébé was then sold on to United days later, Mendes had become entitled to 30% of United's €9m fee, which worked out at €2.7m. The further €0.9m is assumed to be his fee for acting as the agent – a further 10%. Vitória were left with 60% of the fee, according to the reports, €5.4m.

In response to a detailed series of questions about Mendes's earnings on the Bébé deal and Reis's complaint, a spokesman for Mendes's Gestifute agency said: "There is no basis for any action against us in relation to Bébé." She also said: "We never discuss publicly details of the deals brokered by us."

United themselves described Bébé's signing as "one of the most astonishing transfers of the summer," which is an understatement. When the club suddenly bought him on 11 August, Bébé had very little formal football coaching behind him and barely registered as a player in Portugal until Reis negotiated his move to Guimarães, where he played well in pre-season friendlies. Ferguson said Bébé had been recommended to him by his former assistant, Carlos Queiroz, also a client of Mendes and at the time the Portugal manager, and by scouts based in the country. Most extraordinary, perhaps unprecedented, was that Bébé was still living in the Caso Do Gaiato care home, with 80 other boys and young men, until he moved to play for Manchester United.

Reis did believe Bébé had potential; he signed a two-year agreement to represent him as his agent, to run from 19 August 2009. Reis says he negotiated Bébé's first move to the partly professional, financially struggling Estrela, based in Lisbon's north-western outskirts, when Bébé was 19. The club's financial difficulties meant they did not pay Bébé, so he became a free agent. Reis tried to pitch the player's potential to bigger clubs in Portugal and around Europe, including PSV Eindhoven, who he says were not interested. Vitória, based in Guimarães in the north of the country, agreed to take a gamble on him.

Reis says he was negotiating the details of Bébé's proposed contract with Vitória from May last year. Bébé attacked his chance there, playing formidably in the pre-season friendlies. Reis claims Bébé suddenly went missing; he did not turn up to a scheduled meeting in Lisbon and Reis could not get him on his phone. From then, according to Reis, he had no contact with the player he had regarded as his protégé and potentially valuable client. He read in the newspapers that Bébé had signed a contract with Vitória, then, after notable performances, an improved one with the €9m buyout clause.

Bébé, a young man of poverty and patchy education, sent Reis a letter, dated 5 August last year, which said the contract they had was balanced in Reis's favour and that he was unhappy with complaints Reis had made to the Portuguese media about having been kept out of Bébé's negotiations with Vitória. Bébé said he had negotiated that contract himself. For all these reasons, the letter, signed by Bébé, said he was treating his contract with Reis to be his agent as at an end.

Fifa's rules on the conduct of agents, which Sepp Blatter's world governing body still currently oversees although it is about to abandon licensing of agents altogether, state that an agent cannot induce a player to breach his contract with another agent. When Mendes became involved is not clear, nor how he came to represent Bébé, a relationship United confirmed in the only interview Bébé has given so far, exclusively to the club's official website in November. In it, Bébé said he had been playing well for Guimarães in pre-season friendlies and: "Suddenly Jorge Mendes told me about the deal. I was very happy at the time, I didn't expect it. It was like a dream to me."

Reis says he restrained himself from complaining at the time because he did not want to disrupt Bébé's settling in period at United. "What happened was wrong," Reis says. "If he had come to me, and said he is the major agent with the experience of deals at the biggest clubs and we should come to an arrangement, I would have talked to him. Instead I receive this letter signed by Bébé two days before he moves to Manchester United, with Mendes as his agent. That is not right."

Shortly after Bébé joined United, Ferguson lambasted as "vicious" reports that Bébé was too raw a footballer amid the polished skills exhibited by the senior players at Old Trafford. Bébé has made two appearances as a substitute in Premier League matches, played three times in the Carling Cup and once as a substitute in the Champions League, at Bursaspor where he scored in a 3-0 victory. He has also played for Portugal Under-21s, many of whose players Mendes has represented in recent years, along with the majority of the senior national side.

At the time Bébé went to United, Emilio Macedo, the president of Vitória, hailed the work Mendes had done. "Jorge Mendes has been the bulwark of this transfer," he said. "With all due respect to other agents, this country owes him a lot because he handles large transfers and brings money into the country. This is like an export."