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.