Although it's illegal in Spain, there seems to be a way around it the club have found
Catalonia Radio have published a story and SPORT were able to confirm it. Barcelona have introduced clauses into the contracts of loan players known as the ‘fear clause’ in Spain. It means that for all or nearly all of the players they have loaned out, including Nico Gonzalez at Valencia, if he faces Barcelona, then Valencia will have to pay the Catalans some extra money.
It’s a model that Real Madrid stuck to after Morientes shone with Monaco and knocked them out of the Champions League.
Raul de Tomas had it when he was at Rayo, Asensio and Lucas Vazquez at Espanyol, and so on. This is why Collado, we imagine, did not face Barcelona when Elche came to Camp Nou.
It’s illegal to block a player plying against their parent club but this financial element is a way of making it happen.