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 have had the documents since the transfer was concluded in August.”
Pogba is in his second spell at Old Trafford, having left the club for Juventus for £1.5m in 2012.
The France international first joined United from French side Le Havre in acrimonious circumstances in 2009.
He returned to the club last summer for a world-record fee of 105m euros.
United also agreed to pay Juventus 5m euros (£4.5m) in performance-related bonuses, and other costs, including 5m euros if Pogba signs a new contract.
When they confirmed the transfer, Juventus said the “economic effect” to their club was “about 72.6m euros”.