It means Mourinho will set his sights on landing some of the biggest names in the game, including Neymar, Edinson Cavani, Thomas Muller, Paul Pogba and Raphael Varane.
United remain interested in Real Madrid stars Cristiano Ronaldo, Gareth Bale and Sergio Ramos but the Spanish giants are refusing to do business with United.
Van Gaal has ploughed through more than £250m himself since replacing David Moyes in 2014.
But some of his signings have struggled to make an impact, including Memphis, Morgan Schneiderlin, Marcos Rojo and Ander Herrera.
It has also emerged that United will hand Mourinho a contract worth almost £50m to replace Van Gaal.