Paul Pogba’s world-record transfer from Juventus to Manchester United last year is now the subject of a Fifa inquiry.
Football’s world governing body has written to Manchester United”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.
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.
He was allegedly contacted by the BBC, and it was said that Raiola declined to comment and said the matter was in the hands of his lawyers.