The RFEF's Jurisdictional Committee has ruled in Athletic's favour, allowing the club to register Nuin with its Juvenil de Division de Honor side for the current season, after the youngster, nephew of Chelsea defender Cesar Azpilicueta, was signed as a free agent from Tajonar over the summer.
Athletic Club president Jon Uriarte has made a habit of targeting neighbouring academies to strengthen Lezama, and moved for the 16-year-old right-back weeks ago, drawn by his physical power, his versatility as a potential centre-back, and his status as captain of Navarre's under-16 side.
Osasuna had maintained that a two-year contract signed with Nuin's guardians included a clause allowing the club to unilaterally extend his registration until 2028, but Athletic argued the player was a free agent, since he had not yet turned 16 when his previous deal with the Pamplona club was agreed and Osasuna could not tie him down with a professional contract at that age.
"Sergio is a boy who has been here for seven years, who has a contract in force and who has a renewal option that was agreed in the contract, which was signed by them and which nobody forced them to sign. So, we will enforce it," Osasuna's youth football technical director, Patxi Punal, had said after registering him with the club's main youth side.
The dispute escalated further when Nuin was named in Spain's under-17 squad as an Athletic player, a call-up that underlined his rising international profile. Athletic had long monitored his uncle Azpilicueta as a player without ever completing a deal, but have now succeeded in signing a member of his family. Nuin joins fellow arrival Darlington Osuchukwu, a 16-year-old goalkeeper signed from Manchester United after four years in the English club's academy, as one of two names expected to draw plenty of attention at Lezama next season.
Despite Friday's ruling, Osasuna have vowed to continue fighting the decision through Spain's ordinary courts if the federation's committee does not overturn its position, insisting the dispute over one of Tajonar's most promising academy products is far from over.