The report by the Daily Telegraph claims the player has agreed personal terms worth £35,000 per week with Slaven Bilic and is set complete his move.
The Telegraph first broke the story on Wednesday suggesting a £12m bid had been accepted, the broadsheet have since conceded that offer was in fact £18m and now suggest a further £2m of add-ons.
West Ham will retain a 20% sell on clause should the 22-year-old come good with the midlands outfit and be sold.
The Hammers have explained they reluctantly accepted a bid after receiving no other offers for other wingers.
With an unbalanced squad, the money will be used to reinforce defensive positions with a defensive addition expected to be announced within the next two weeks.