Red Devils boss Jose Mourinho is desperate for first-team stars De Gea and Martial to sign contract extensions at Old Trafford.
The duo have been locked in talks with club chiefs for several months but Manchester United are finding it difficult to get the deals over the line.
And the Daily Mirror say Uniteds prospects are being hampered by agents using Sanchezs salary as a guide in negotiations.
Manchester United signed Sanchez from Arsenal last January and handed the Chilean a stunning salary of over £400,000-a-week.
It is claimed Sanchezs teammates at Old Trafford are now trying to use this to their advantage, with De Gea said to be eyeing £300,000-a-week terms in a new deal.
De Gea has developed into one of the best goalkeepers in the world since joining United and knows his importance to the team and Mourinho.
The Red Devils are also struggling to tie Martial down to a long-term contract, with the Frenchman reportedly rejecting an offer to extend his stay at United last month.
Martial has netted five goals in his last five games and is another player who is said to be looking for some kind of parity with Sanchez, who he is outperforming this season.
Ahead of Saturdays trip to Bournemouth, which ended 2-1 to United, Mourinho urged Martial to stay at the Theatre of Dreams.
He said: I hope we will agree. He is a talented player – everybody has known that since he was at Monaco.
Another thing is a talented player with consistency and understanding what a football match is in the globality.
When you reach that level or are almost reaching that level is when you become a top player. One thing is a top talent, another thing is a top player.
When he is now much closer to becoming a top player, obviously I would like him to stay. I know the club very much wants him to stay so hopefully he is going to stay.