If they are miles apart sell in the summer take the cash and re-invest it elsewhere.
This is a direct consequence of the Sanchez deal you can't put the top earners on that much money when they haven't earned that deal at the current club sends the wrong message to the dressing room.
Martial should be offered a decent increase on his original salary but with good incentives if certain targets are met each season.
If he goes on the market this summer even with only 12 months on his contract I don't think he'll get offers better than what Utd would offer him. He'd be joining another "current" second tier team while he's got all that talent the lack of consistency means he'll be seen as to much of a gamble for the very top sides to get to those clubs you need to do in 8 out 10 games not 5 out of 10 which is sadly where Tony Martial is right now.
Like @jsp eluded to, Tony can turn up and be class, brilliant pace and turn of direction and a good eye for goal. But he doesn't do it enough, could you say that under a new coach he will progress ? I would argue yes he would improve, but very hard to say ...
Think a lot of our problems stem from the fact Sanchez has come in on £350k- £450k a week and its peed off a few as Sanchez has been a massive disappointment.
You just don't know you compare him to Sane who came in at a similar time he's got a few "attitude" problems to but he seems to maintain much more focus that Martial does.
For me Martial lets his head dip to quickly maybe with a different coach he might fill him with more confidence but right now he plays like someone who's terrified of being subbed he doesn't play with confidence and freedom.
It's easy to hammer the ex manager over the development of these players but they're all given plenty of chances and very rarely do they force themselves to be picked the consistency just wasn't there.