Lee made his debut for Barcelona B during the 2015-16 campaign, but he represented their Juvenil A team last season.
It is understood that the teenager has become frustrated with a lack of opportunities at Camp Nou, and his agent Guardiola has suggested that a move to Dortmund could occur in this summer's transfer window.
"Dortmund have enquired about Lee with Barcelona. Talks are ongoing," Bild quotes Guardiola as saying.
Lee, who is understood to have a £2.5m buy-out clause in his Barcelona contract, moved to Camp Nou from South Korean side Incheon United in 2011.