West Ham are investigating the deal that saw Arthur Masuaku move from Olympiakos in the summer.
West Ham club officials are concerned they overpaid for the French defender by almost £1m.
The Hammers are trying to clarify whether someone who was not declared on the official transfer document was paid.
A representative was sent to Athens last week to establish if that was the case.
The Premier League and Football Association have been contacted to comment but are yet to respond.
Masuaku came through the youth system at Valenciennes in his home country before joining Olympiakos in August 2014.
He played 65 times for the Greek side before completing the deal to join West Ham that has now caused concern.