Portuguese TVI 24 journalist Rui Pedro Braz is reporting Sporting are demanding a £55m up front fee for the midfielder.
But United are willing only to pay £42m - and the £13m disparity is reportedly putting the protracted move "at risk".
Braz says Sporting are insisting on a guaranteed fee of £55m plus bonuses.
He claims United - who were reportedly looking to have Fernandes watch United against Liverpool at Anfield on Sunday - are unwilling to meet that cost.
Braz goes onto add that United have offered 25-year-old Fernandes massive bonuses.
But Sporting feel these are unrealistic, especially as they are believed to include him getting a bonus for winning the Ballon d'Or.
Another two bonuses are for winning the Premier League as well as the Champions League.