West Ham are investigating the deal that saw French left-back Arthur Masuaku join them from Greek champions Olympiakos over the summer.
The Hammers paid £6.2m to land the 22-year-old but it is understood club officials are concerned they overpaid for the player by almost £1m.
West Ham are concerned someone was paid who was not declared on the official transfer document, with a representative dispatched to Athens last week to establish if that was the case.
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.
Masuaku has played in all four Premier League games for Slaven Bilic's side this season.