Manchester City paid £41.4m for the transfer of Eliaquim Mangala in 2014, according to a leaked document.
The deal to sign the Frenchman from Porto was originally reported to be £32m but the eventual cost was much higher due to the player's complicated ownership arrangement.
The document, which was mysteriously leaked online on Wednesday, revealed that Porto only owned 56.67% of Mangala's rights and were paid a sum of £23.5m for the transfer.
A further £13.8m went to The Doyen Sports Investment Limited, who had 33.3% of the rights, while the remaining £4.1m went to an Austrian marketing company for their 10% stake in the player.
The fee makes Mangala, who has featured 14 times in the league for City this season, the record British signing for a defender.
