Mitchell, 26, is provisionally lined up to fight European lightweight champion John Murray in April.
But he is yet to agree to the purse being offered by Frank Warren.
The BBC understands that he is being offered less than half the fee he got for his last fight - a defeat by Australian Michael Katsidis.
"Kevin is tied in to a five-fight deal with Frank Warren, which means he can't go and box elsewhere. He won't accept the fight unless the money is dramatically improved, which could leave him with nowhere to go," a source close to the fighter told the BBC.