Some people argued hard that players like Sanchez should get paid higher because of the lower transfer fee while others on higher transfer fee the salaries should be lower? Where is that logic come from? Just ridiculous! The transfer fee is paid to compensate the other entity, not the player involved in the trade. Player's salary should have nothing to do with the transfer fee. Player's salary should be calculated by factoring in the quality, experience, demand, supply and impacts to the team.
The salary issues on quality player like DDG or Pogba were going to come up the moment Sanchez was getting overpaid because the quality and experience are very much inflated by the new standard that applied to Sanchez. So forget how other players from other team are getting paid, every United player's agent is sizing up client's salary based on THIS TEAM's standard, not other teams or overall market.
Someone also argued that because of Real got a goal keeper so DDG is not on high demand. True. But you still don't want our arguably best player not happy. Even DDG had shown before he was very professional over things not going his way, but everyone in the club house knows there is an elephant in the room, you cannot hide it. Not only DDG, other players may also think, "this is how I get treated?, where is the love?"
So since we cannot rewind the Sanchez's deal, the solution is pretty clear, just pay DDG, and prepare to overpay quality players in the future.