I don't believe the buyback is mandatory. So Gerard would have to agree to a new contract with Barca, which he could refuse. That's what I base my assumption on.
If it's not mandatory then I'm not really worried. He loves playing for us and we love him plus he has a manager that believes in him. One day he might go back to Barca if he's good enough but if he has the choice I don't think it will be in the next couple of years.
It's not necessarily how contracts work. He could have agreed to it as part of the deal to allow him to leave.