Leeds are expected to lose a plethora of players following their relegation from the Premier League and Aaronson is one of them.
The United States international joined the club last year but he is not keen to play in the Championship next season and an escape route is in the offing.
A move to Germany is expected to happen as the player closes in on leaving Leeds in the ongoing transfer window.
Union Berlin are the club who are set to sign the Leeds midfielder on loan and an agreement between the two sides is close.
The Bundesliga club are in advanced negotiations with Leeds to secure a loan deal to sign the 22-year-old.
Several English and German clubs were interested but Aaronson has decided to opt for a move to Union Berlin.
They finished fourth in the Bundesliga last season and will offer the US international a platform to perform in the Champions League.
It is unclear whether Union Berlin will have the option to buy the player from Leeds as part of the loan agreement.