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.
The France midfielder 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 plus 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".
A book published in Germany this week - The Football Leaks: The Dirty Business of Football - and reproduced in media reports, includes what it says is a breakdown of the Pogba fee and alleges his agent Mino Raiola earned £41m from the deal.
When contacted by the BBC, Raiola declined to comment and said the matter was in the hands of his lawyers.
Were there any more details?
According to reports taken from information in The Football Leaks:
- Forward Zlatan Ibrahimovic, another Raiola client, earns £367,640 a week - £19m a year - at Manchester United, making him the best-paid player in the Premier League.
- Pogba's basic salary is £165,000 a week - £8.61m a year - but he has substantial incentives in a 41-page contract.
- Raiola took a £23m slice of the transfer fee and will be paid five instalments totalling £16.39m from United over the course of Pogba's contract.