Ordinarily the Shrews would not be entitled to a fee because Humphrey has joined a team from another country.
But manager Paul Simpson told BBC Radio Shropshire: "It's now in the hands of Fifa. We could be looking at a hefty sum, but it's what we're entitled to."
Simpson added: "Just because we're in League Two doesn't mean a Scottish Premier League side can walk in and cream off our best players.
"We've got letters from the Football League, from the Football Association and from the Professional Footballers' Association all supporting our case."
Humphrey joined Shrewsbury from West Brom in September 2006 and made 88 appearances for the Shropshire club.
Motherwell have refused to comment on the situation.
Source: BBC Sport
Source: BBC Sport