Zatara
Player Valuation: £100m
More common than you'd think. When "options" are agreed in a deal, it's not uncommon at all for a player to have to agree a future contract as part of the deal - this gives the buying club assurances that someone isn't going to come in and gazump them on personal terms of they have a successful loan spell.
Another example, albeit in a slightly different scenario, Deulofeu when he was here the 2nd time around and Barca held a buyback clause, the contract was pre-agreed - Deulofeu effectively had no say in it if Barca exercised the option because he'd already signed a deal which committed him to a contract if they did it.
Same as Yerry Mina as well.
Next to 0% chance he would play for Barca again. But all had to be agreed before we signed him.
Barca also sign players they have sold using their options. Those players get game time elsewhere, prove themselves and go back to Barca.