Fifa is investigating the transfer of Eliaquim Mangala from Porto to Manchester City in August 2014.
The 24-year-old joined City for a fee understood to be about £32m and signed a five-year deal.
The France international's move is being looked into to see if it breached third party ownership rules at the time, according to Bloomberg,
The report added that Porto's role in the transfer is under scrutiny rather than City's actions.
City are not aware of any investigation and Premier League rules forbid the involvement of third parties in transfers.
Mangala began his career with Standard Liege in 2008, winning the league in his first season, before joining Porto in 2011.
He won two Portuguese titles and made his France debut in a 1-0 defeat by Uruguay in June 2013.