The transfer of Paul Pogba from Juventus to Manchester United is now the subject of an inquiry by Fifa.
The midfielder was signed by United for a record-breaking 105 million euros (£88m) last year, with more money agreed for performance-related bonuses.
Fifa's inquiry is thought to be about who was involved in negotiating the transfer and how much money those individuals received. The role of Mino Raiola, Pogba's agent, has been the subject of controversy after it was claimed that he could eventually make up to £41 million from the day.
The world's governing body for football has now written to the club, requesting clarification on the deal which was agreed between the two clubs.
A spokesperson for Man United refused to comment on the contract to the BBC, and made it clear Fifa have had all of the necessary documents since the transfer was finalised last August.