I just supposed they already set the terms of the contract he would then be signing or just extend his current contract/convert it into a permanent Sevilla contract. They don't have to "force" him as he will already have been at Sevilla for 2 years so it's not like, for example, a club paying a player's release clause and forcing him to play for a team he doesn't even know.
Otherwise, the 9m option we have seems pretty redundant to me. I'd get the logic behind it but it would still seem weird; if he's good enough for Barça to pay 9m for him then he wouldn't agree to a move to Sevilla anyway and if he did, he would just cost Barça 3m for nothing. Add to that his supposed insistence on Barça's option and that logic becomes even weirder.
The way I see it, when they negotiated his transfer they also set the terms under which he would play for either of the teams afterwards, so technically, he could be under contract for either of the clubs depending on who pays more.