"Option" could mean many things.
It's entirely up to them to decide.
They've no choice - they have to pay it whatever happens.
They have to pay it if certain targets are met vis-vis performance.
This is the purchase price they're obliged to pay, but we have the option of accepting a higher bid from elsewhere , should one come in
Anyway hope he performs so well that AS Roma buy him next summer.
As the rhyme goes: When he was good, he was very very good. When he wasn't, he was ... ...