You never know for sure, but there is some discussion of this here:
https://www.eurosport.co.uk/footbal...ed-s-contract-revealed_sto6882351/story.shtml
So he gets bare minimum £411,000 per week (£391,000 wage plus £1.1 million signing on fee divided by 52). Then he's basically guaranteed to get the appearance fee, because you're not going to pay someone £411,000 per week to do nothing. Then there will be other bonuses.
Ultimately, you're looking at a £500,000 per week wage. And he's got nearly 4 years left on his contract. So he will earn in the region of £100 million in the remainder of his United career if he sees out his contract.
So if United wanted to buy out his contract tomorrow, they'd have to offer him at least half of that. It would be a bare minimum £50 million. If you could offer him that, and he could get a contract worth £250K per week then he'd be as well off as he is now. So in order for United to buy out his contract, you'd need a bid of £50 million + tax. So £60 million. That's just so United could have a chance of buying his contract out, and breaking even.
If someone came in and offered £60 million, and a big enough contract to Sanchez, and he was agreeable to it, and all issues could be resolved then United could get rid of him. But what they would have effectively done is released the best paid player in the PL on a free transfer. Admittedly, this would free up some wage budget, but you'd have a hard time selling this to the money men, I fancy.
The numbers will evolve as his contract runs down, but he will also be 30 by the time of the next transfer window.
Also, I don't want to sound sanctimonious, but those sort of numbers are the reason why I've gone off football to a large extent. The gulf between owners, players and fans is a chasm, and I wouldn't want to put any more money in their pockets than I do by watching on TV. And I'm dubious about whether I want to continue doing that.