I'd only sell if we are guaranteed to sign Mbappe or Aguero otherwise I'd force him to stay. Transfers can be unpredictable and a year is a long time in football, we have seen players have a last minute change of heart. Wasn't Angulo on his way to London to sign for us before he made a complete U-turn and stayed at Valencia ? De Gea looked set to sign for Madrid few seasons ago but he's still at Man United and he seems pretty content there these days.
Unfortunately I don't trust this club to turn down 50m bid for Sanchez though, especially from a foreign club, knowing we may lose him for free in 12 months time. Let's see if we are finally ready to put sporting ambition ahead of money