Just sell him, I see no reason to play him/improve him/give him more experience all for making him a better player when Barca snaps him up for free next summer.
We should improve/put energy into players that are here to stay.
I think if that 18m euros is the best Txiki was able to negotiate, we have to accept.
We don't have to accept Anything.
Barcelona have more to lose not signing him this summer, than we do in not taking a below offer valuation. Up to them to pay to sign him. Otherwise, he seems happy to stay, the club seems happy to keep him around, so Barcelona can wait. Or, they spend even more on another defender that isn't a 'catalan cantera kid' and has no political PR value.