Eliaquim Mangala's transfer to Manchester City from Porto last year 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 last year - Porto receiving €30.5million for their 56.67 per cent 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.
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