Sporting Lisbon have reached an agreement with Nuno Mendes to increase his release clause.
He recently signed a new five-year deal with Sporting, which included a €45m release clause.
But according to Portuguese newspaper Record, Sporting have now raised that amount to €70m.
The fee is a significant amount of money for a teenager with just 13 first-team appearances to his name.
United reportedly attempted to include Mendes in their deal for playmaker Bruno Fernandes.