If he's on 325K then after this season he's due another 50m from the club. I think the will of the player is the biggest stumbling block here. I don't see the elite clubs coming in for him. But clubs like a Bournemouth or Villa could rejuvenate him and get some performances out of him. Would he go to those clubs? And realistically how much would they pay him max, 150K? I think that even that would be a stretch, but for the sake of argument, let's say that's the maximum. And he won't cut his own future earnings, so United would have to subsidise his wage for the next three years, at a cost of 27m or so. Selling him for 40m. Making a profit of 13m. I'd do that deal, but it would mainly benefit the club by getting rid of his negative influence (a passenger on the pitch, and his PR team off it, and a massively overpaid player in the dressing room) and it would give us great leeway with regards to PSR.
I don't know how creative you can get to find a deal acceptable to him and another club. If he's interested in Saudi, everybody wins, but if not, it's incredibly complex.