Eliaquim Mangala's transfer to Manchester City from Porto in 2014 is being investigated by FIFA amid suggestions the switch contravened third-party ownership rules.
Mangala moved to the Etihad Stadium from the Primeira Liga side in August - Porto receiving $47.8million for its 56.67 per cent ownership of the player's rights.
While third-party ownership was banned by FIFA in May, private equity fund Doyen Sports had invested in Mangala prior to his departure for England.
"We are looking into this," a FIFA spokesperson told Omnisport when asked about the transfer.
Third-party ownership was particularly common in Portugal, with Doyen having also invested in Manchester United defender Marcos Rojo - formerly of Sporting CP.