The Uefa president, Michel Platini, had no written contract for the 2m Swiss francs (£1.35m) Fifa paid him in 2011, which is the subject of criminal proceedings in Switzerland and a Fifa ethics committee investigation. According to sources with knowledge of the payment, Platini and the Fifa president, Sepp Blatter, are understood to have told investigators the money was paid due to an agreement they made orally when Platini worked as Blatter’s adviser at Fifa, between 1998 and 2002.

Platini, who has made four statements about the payment but given no detailed explanation since the Swiss attorney general, Michael Lauber, announced it is subject to criminal proceedings, has said Fifa did not pay him in 1998-2002 because of the organisation’s “financial situation at the time”.

Yet even if such a payment was validly due, despite there being no agreement in writing to document it, and Fifa did genuinely struggle to pay it in 2002, Swiss law limits to five years employees’ claims for money owed. This perception – that the payment was in fact time-barred and therefore no longer legally owing – is understood to form part of both Lauber’s criminal proceedings and the investigation by Fifa’s ethics committee, which last week suspended Blatter and Platini from football activities for 90 days.

Lauber, when announcing the criminal proceedings against Blatter last month, said the payment is suspected of having been “disloyal” to Fifa – meaning a breach of Blatter’s position of trust as president – potentially amounting to “criminal mismanagement” or “misappropriation”.

Platini, Lauber said, was interviewed as a “person asked to provide information” - not as a witness, as Uefa said in its initial response. That is a legal status in Switzerland between being a witness, over whom there is no suspicion of wrongdoing, and a suspect, and it allows for the possibility the person questioned may become a suspect, or be cleared, as an investigation proceeds.

Fifa’s ethics committee is understood also to be investigating whether the payment breached the requirement in Fifa’s ethics code for football people to avoid “existing or potential conflicts of interest”. Platini received the £1.35m in February 2011, shortly before he decided not to stand as a rival candidate in the May 2011 presidential election and instead supported Blatter.

In Platini’s first public statement, on the day Lauber announced the criminal proceedings, he explained the payment “relates to work which I carried out under a contract with Fifa”.

Then in a letter sent to Uefa’s 54 national associations, including the English FA, which continues to support him, Platini did not explicitly mention a contract, instead saying the money related to his “full-time job” at Fifa between 1998 and 2002 and “the remuneration was agreed at the time”.

The Guardian understands Platini did have a normal Fifa contract of employment during his years working as Blatter’s adviser, which paid him a salary of 300,000 Swiss francs (£203,465) a year. He is understood to have worked in the Fifa job for three and a half years and been paid 1.05m Swiss francs (£712,000) in total under that contract.

Platini joined the Fifa executive committee, which pays allowances, the year he stopped working for Fifa full-time, 2002, and worked in other football advisory roles until he was elected Uefa president in 2007.

When the 2m Swiss franc payment of February 2011 was brought to Lauber’s attention earlier this year, Blatter and Platini are understood to have said it was due according to an oral agreement. Platini is understood to have told investigators he had agreed with Blatter, in 1998-2002, that he would be paid 500,000 Swiss francs (£339,000) annually on top of his contractual 300,000 (£203,465). The 2m Swiss francs appears to be explained as four years’ worth of payments of that 500,000.

Part of Lauber’s investigation, and that of the ethics committee, whose two “investigatory” and “adjudicatory” arms are chaired respectively by the Swiss criminal lawyer Cornel Borbély and the Munich judge Hans-Joachim Eckert, is whether Blatter had authority to make such an oral agreement.

In his letter to Uefa’s associations Platini offered no explanation for the delay until 2011 in the 2m Swiss francs payment, saying only it was “the final outstanding amount” due from his employment with Fifa. Then Platini told the news agency AFP the reason for the nine-year delay was that Blatter “informed me when I started my role as his adviser [in 1998] that it was not initially possible to pay the totality of my salary because of Fifa’s financial situation at that time”.

Platini said he did not pursue it until 2011, when he requested that the “outstanding balance” be paid. “The fact this payment was made a few months before the Fifa presidential elections is irrelevant,” Platini said, “since I never had any plans of becoming a candidate.”

Fifa’s accounts for the 1999-2002 period – the financial statements are collated for the four years between World Cups – show a total income of 2.7bn Swiss francs (£1.8bn), and an overall profit of 115m Swiss francs (£78m). The organisation did record a significant loss in 1999 but that was followed by strong profits in the next two years.

In 2002 the total wage bill for employees was more than 42m Swiss francs (£28.5m). The official Fifa statements accompanying the 1999-2002 figures stated that despite the collapse of the marketing company ISL “Fifa was able to take the correct steps to ensure that every obligation [sic] was fulfilled on time and in its entirety.”

Even if there was an oral agreement between the two football chiefs which meant the money was validly owed, and even if Fifa did genuinely struggle to pay, both Lauber’s and the ethics committee’s investigations are understood to be focusing on the fact that employees’ claims lapse after five years under Swiss law. Hence there is serious doubt over whether a payment in 2011 can have been legally required to be paid for work done which finished in 2002.

The ethics committee’s suspensions of Blatter and Platini have been made on the basis there is a prima facie case of wrongdoing, which needs to be fully investigated without any possibility of interference by the men. Both have appealed against their suspensions to Fifa’s appeals committee, chaired by the former Bermudan attorney general and president of the Bermuda FA, Larry Mussenden.

Platini has insisted he will clear his name of all the allegations, describing them as “based on mere semblances” and “astonishingly vague” while Blatter’s lawyers, the American Richard Cullen and Swiss Lorenz Erni, said he will prove he “did not engage in any misconduct, criminal or otherwise”.

Both Platini and Blatter declined the opportunity to respond to the Guardian on the details of the 2m Swiss franc payment. Cullen said they were not prepared to discuss the facts outside the committee processes. In a statement on behalf of Platini a Uefa spokesman said: “The president is cooperating fully with the investigations and feels there is nothing more he wants to say publicly for the moment.”