I think it would be very difficult to convince a player and their agent to do that as that would really screw a club up for the remainder of the season pretty much poaching their keeper without giving them a chance to replace him
IIRC a team whose player we would sign outside of the regular transfer window by paying his buyout clause would have 30 days to buy his replacement (don't know the rules exactly but I think it has to be from another Spanish club) if they would decide to to so.
But in this case (Valdes was injured in late March) our options were probably limited to getting a backup GK from another team who would be better than Pinto. And if we would find one he would still need time to adapt and you can't experiment with GKs so late in the season. That's why we probably decided to stick with Pinto.
Anyway, whichever GK we could potentially sign at that time would be a clear downgrade from Valdes who was probably a top 3 GK in the world before the injury.