Michel Platini's lawyers say a memo published in a French newspaper on Sunday could help clear the suspended Uefa president of wrongdoing.
Platini and Fifa president Sepp Blatter have been suspended over allegations a "disloyal payment" was made to Platini in 2011 for work done years earlier.
Le Journal du Dimanche says the memo, dating back to 1998, includes a payment agreement between Platini and Fifa.
Platini's lawyer Thomas Clay called the 23-page document "important evidence".
Clay, speaking to the Reuters news agency, added: "From the moment that we have proof of an agreement between Fifa and Mr Platini, and of knowledge of this agreement by officials of Uefa, then this inquiry falls down.
"For us, it's very important evidence that Mr Platini has always been telling the truth."
Both Platini and Blatter are fighting to salvage their careers and reputation and are currently serving 90-day provisional suspensions from world football.
It is alleged a £1.3m "disloyal payment" was made between Platini and Blatter in 2011.
The two men deny wrongdoing and claim they had a verbal contract for Platini to receive backdated money for work he carried out for Fifa between 1998 and 2002.
Le Journal du Dimanche claims the memo, which has not been seen by BBC Sport, was presented at a meeting of Uefa's executive committee in November 1998 and included a section that stated Platini was being paid 1m Swiss francs a year for work he was doing for Fifa.
"It shows that the contract did not have any sort of secret character and that many people, including those in Uefa and Fifa, have known about it since 1998," added Clay.
Blatter has previously said the 2011 payment followed a verbal agreement between the two while Platini claimed the payment was delayed because of financial problems at Fifa.
Both men are due to attend personal hearings with Fifa's ethics judge starting on 16 December and final verdicts could be published as early as 21 December.
Platini, who had been widely tipped as Blatter's successor at Fifa, still hopes to be a candidate in the Fifa presidential election on 26 February.
However, he will only be entitled to enter the ballot if he clears his name in sufficient time prior to polling day.