Fifa president Sepp Blatter's payment to Uefa chief Michel Platini was a "conflict of interest", says Domenico Scala, chairman of the world governing body's audit and compliance committee.
Swiss Blatter, 79, and Frenchman Platini, 60, are suspended while Fifa investigates reports that a £1.35m payment was made in 2011 for work Platini did as Blatter's adviser.
Platini claims the deal was an "oral contract" for work done from 1998-2002.
Both men deny any wrongdoing.
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Scala told the Financial Times that the payment was not recorded in the accounts in 2002 and called that a "serious omission".
Blatter is stepping down as president after 17 years, with Fifa confirming on Tuesday that it would elect his successor on 26 February 2016.
Fifa vice-president Platini maintains he still wants to stand for the next presidency, but Fifa says it cannot recognise his candidacy while his ban is in place and he cannot campaign.
"In any normal business, there would be a written contract," said Scala. "Both parties were members of Fifa's executive committee and knowingly approved each year financial statements which were incorrect by 2m Swiss francs (£1.35m).
"That could be seen as falsification of the accounts."
The payment was made in February 2011. Platini later ruled out standing against Blatter in that year's Fifa election, saying he wanted to concentrate on running Uefa.
Both men are appealing against their "provisional" 90-day suspensions - put in place while Fifa's ethics committee completes an investigation into two of the game's most powerful figures.
Scala is also head of the ad hoc election committee which rules on the eligibility of presidential candidates, but he has no say in the ethics case.
On Wednesday, Fifa's independent ethics committee will discuss pending proceedings against senior figures under investigation.