Barcelona loaned Fort to Elche last season, where he scored two goals and added two assists in 14 La Liga appearances, seven of them starts, having made 17 appearances the previous season, five as a starter, for the first team.
He returned to pre-season training with Barcelona this summer but has continued to be linked with a fresh move, either permanent or another loan, amid a squad packed with options in defence.
Sky Sport Germany reports that Dortmund have now moved beyond monitoring the 20-year-old, holding initial talks over a transfer and gaining an understanding of the terms required to get a deal done.
Barcelona are believed to be seeking between €15m and €20m for the full-back, depending on how much of his economic rights they retain, though that figure could fall closer to €10m if the club keeps hold of half his rights rather than selling outright.
Fort has also been linked with Premier League interest from Everton, Fulham and Liverpool, as well as Racing Santander in Spain, but Dortmund's growing involvement suggests the Bundesliga club see him as a genuine alternative to Yan Couto, who left Dortmund for Como on loan earlier this summer.
Barcelona want the situation resolved quickly, with the versatile right-back having found his path to regular first-team football blocked by Jules Koundé and other options in Hansi Flick's squad.
A La Masia graduate who has been at Barcelona since the 2013-14 season, Fort made his first-team debut in 2023 and has 30 competitive appearances for the club to his name, though he has never managed to establish himself as a regular starter.
His versatility — capable of playing at right-back or further forward on the right flank — has made him an attractive target for clubs across Europe, with at least ten teams reported to be monitoring his situation as the summer window enters its final weeks.
Barcelona, in line with their recent approach to selling La Masia talent, are expected to push for some form of ongoing control over his future, whether through a buy-back clause or performance-related add-ons, whichever club ultimately wins the race for his signature.