Paul Pogba’s world-record transfer from Juventus to Manchester United last year is the subject of a Fifa inquiry.
Football’s world governing body has written to the Premier League club “to seek clarification on the deal”.
It is believed to concern who was involved in the £89.3m transfer, and how much money was paid to them.
A United spokesman said: “We do not comment on individual contracts. Fifa has had the documents since the transfer was concluded in August.”
Pogba, 24, is in his second spell at Old Trafford, having left the club for Juventus for £1.5m in 2012.