Record says negotiations over the midfielder's proposed switch to Old Trafford have stalled this week.
A stipulation in Fernandes's current contract entitles him to a £5million bonus - to be shared with his agent Miguel Pinho - should Lisbon receive an offer worth at least £50million for his services.
United are said to have lodged a bid of that amount as a guaranteed up front payment, with additional fees dependant on performance-related bonuses.
However Lisbon have now argued the add-ons are unrealistic, and want to push for a deal starting at £60m as an initial figure.
Sporting president Frederico Varandas has told United he is not prepared to let Fernandes go for less than that amount, but is willing to sanction a sale should the Red Devils increase their offer.