Manager Michael Carrick has already given his approval to sign Salinas, 19, should the pursuit of Hall fall through, with Manchester United needing reinforcement at left-back given Luke Shaw, 31, has entered the final 12 months of his contract at Old Trafford.
Newcastle United have already lost Anthony Gordon and Sandro Tonali this summer, and are set to lose captain Bruno Guimarães to Arsenal too, making a fourth departure unthinkable for a club unwilling to strengthen a direct Premier League rival; that resistance has pushed United to look elsewhere.
Salinas, born in Santander and with Racing since 2019, impressed United's scouts at the Under-19 European Championship, where Spain won the title, and can play at left-back or centre-back, having been described in some reports as a "left-footed Sergio Ramos."
Barcelona have also been tracking him for some time but are reportedly only willing to offer around €6-8m, split between a fixed fee, add-ons and a player makeweight, well short of the full clause; Mundo Deportivo claims United are ready to pay the €16m in full to cut Barcelona out of the deal entirely, even though the player himself is understood to favour a move to Catalonia.
There had also been earlier interest in versatile Arsenal defender Myles Lewis-Skelly, but it's understood that avenue will not be pursued this summer.
United are still assessing their priority position for the closing month of the window, with reinforcements also needed in attack, but the Salinas interest suggests left-back has become a live alternative should the more complicated Hall deal fail to progress.
Should United act, Racing Santander would have little choice but to accept once the clause is paid in full, leaving no room for further negotiation on their side.