The 19-year-old is to seal a permanent switch to the Bundesliga club, subject to international clearance, ahead of tonight's deadline for the January transfer window.
The Norway Under-21 international joined the Reds from Tromso in his homeland in 2020.
Hansen-Aaroen has appeared regularly for United's Academy sides and gained experience in the UEFA Youth League, playing in the away games at FC Copenhagen and Galatasaray this season.
The next chapter of the teenager's career is going to be in Germany, and everybody at United would like to wish Isak the very best of luck for the future.