He did try to get Sterling last year. Unfortunately, Pep knew he was better off waiting. Mourinho, on the other hand might think that he is better off getting Sanchez now than waiting for him to go into the free market and might be more willing to allow one of his myriad forwards to leave. If that forward is interested in joining the Farcenal of the future that Wenger is building.
I would take Martial. If you get Martial in this deal, run along and bring in Mahrez now, I suspect you have a basis to beg Ozil, even though he would be mad to sign anything before June.