Warning: high level math to follow, courtesy of the Daily Star. Hope you have your thinking hats on. No, not your magic hats. Those really don't work as well as advertised nowadays.
Jose Mourinho is barely three months into his new four-year deal, with a yearly salary estimated to be £10m. Now this next bit is tough, so pay attention. Should he finish out all four years, he would earn £40m. Technically this is gross — not gross like eeeww how dare he make so much money, but gross as in before taxes — but that's a non-basic detail and we're strictly talking basics here. In case I lost you on how we arrived at the £40m, it goes a little something like this: £10m per year, for four years, is 4 times £10m, which, according to my calculator is £40m. Whoa. Texas Instruments TI-89 Titanium (i.e. the T.I. TI-89 Ti), blessed be thy name, your kingdom come, your will be done. Enter.
Since we're roughly a quarter of the way through the year (3 months out of 12 months is 25% of the months), Mourinho has already earned £2.5m of the £10m due to to him this season. One quarter of 10 is 2.5. 25% of 10 is 2.5. See how the numbers match each other? That's how you know you're doing amazing work. Give yourself a gold star and a bee stamp.
Final step, class, who can tell me what is £40m minus £2.5m? If you said £37.5m, then I shall call you a cheater for clicking the link above to the Star's article, which already revealed this answer. Cheater. No one else could've gotten this answer. Thank you, Daily Star, for sciencing the [shoe] out this.
So there we have it, if or when Mourinho gets fired,* he will be due the unpaid but guaranteed portion of his contract. Just like many other managers before him. In most cases in world football, this will mean the full amount — it's not like the NFL where, in general, only signing bonuses and small percentages of the actual contracts are guaranteed money. ESPN's Gab Marcotti does claim that there's an early exit clause built-in to Mourinho's contract (hardly surprising), which would guarantee only half the full amount. Either way, it would be a lot of money and most likely, a world record severance paycheck.
But it won't matter because he won't get the sack and we'll all live happily forever after. Right? Right.
* If a manager leaves voluntarily or by "mutual consent", then different rules apply with generally much lower if altogether non-existent severance payouts (which may or not be pre-negotiated). In most cases, if a manager takes a job before a specified period of time passes, he foregoes the unpaid amount still due to him under the terms of his severance (see: Andre Villas-Boas, who took the Spurs job just 5 months after getting the sack).