That could be £300m, if they're under no obligation, it's worthless. Why would Everton pay £50m for a player with 12 months left on his contract when they had him for a year on loan with 2 years left. The very inclusion of that, if true just makes us seem amateurish.
Option to buy is only ever a worthwhile clause for the team bringing in the player. it protects them against inflated prices if the player performs whilst with them, effectively punishing them for getting a tune out of a player. when they're included by the selling club, they're useless and nearly always are either passed on or are renegotiated.
I dont see a scenario where Everton are pushing for a clause that would allow them to buy grealish for £50m next year with 12 months left on his contract when they could have him for less now or get a cast iron obligation to buy in 12 months for less too which suggests it's city that have pushed for that if it is indeed accurate.