The 23-year-old spent four seasons with the Bundesliga team from 2017, registering 50 goals and 64 assists, before completing a transfer to Old Trafford in the summer of 2021.
Sancho has made 82 Reds appearances so far, scoring 12 goals across all competitions and winning the Carabao Cup in 2023.
Sancho will now return to Germany on a temporary basis as Borussia Dortmund prepare to resume their Bundesliga campaign after a winter break.
The England international of 23 caps leaves with our best wishes for the remainder of this season.