West Ham midfielder Declan Rice's contract carries a buyout clause.
United have been dealt with a major blow in their bid to sign Rice, who has also been linked with Chelsea.
The Daily Mail reports United have become increasingly concerned over Rice's release clause, which is said to be in the region of £100million.
Hammer boss David Moyes recently said: “I don't agree with the owners that Declan Rice is a £100m footballer.
“Far, far more than £100m"
