The deal is not yet finalized. Aaronson still has to undergo a medical and finalize small details. Aaronson will remain with the Union through the end of the 2020 season before joining Salzburg.
Philadelphia declined to comment.
Aaronson, 19, broke into the Union first team in 2019 and has been a constant in the starting XI this year. He has six goals and five assists in over 40 MLS regular-season appearances and earned his first call-up to US men's national team in October.
Aaronson's MLS highlight reel:
A modern midfielder, Aaronson has thrived in the Union's high-pressing system. Bild dubbed Aaronson the "US Kai Havertz," referring to the highly-rated German star who recently completed a big-money move from Bayer Leverkusen to Chelsea.
RB Salzburg is one of Austria's biggest clubs and is coached by American Jesse Marsch. The former New York Red Bulls manager guided Salzburg to their seventh consecutive league title last year, as well as their second consecutive Austrian Cup triumph. It was his first season in charge.
A move to Salzburg could also see Aaronson take part in elite European competition. Marsch's side is in a strong position to reach the Champions League group phase after winning their playoff round first leg on Tuesday.
There are plenty of connections between the two clubs as well. Union sporting director Ernst Tanner previously worked for RB Salzburg before joining Philly, while Marsch and Union head coach Jim Curtin played together at both Chivas USA and the Chicago Fire during their playing careers.
As previously reported by MLSsoccer.com and others, Aaronson was on the radar of many European clubs, including Hoffenheim, Celtic, Anderlecht and others.