West Ham United have launched an investigation into the £6.2million deal that saw Arthur Masuaku move to the club from Greek champions Olympiacos in the summer.
Senior club officials are concerned they have overpaid by almost £1m for the 22-year-old French left back and sent a representative to Athens last week to establish if that is the case.
West Ham are trying to ascertain whether someone has been paid who was not declared on the official transfer document.
