Non-Priestly Blessing

Rashi explains (9:22) that the beracha Aharon said was birchat kohanim.  The Ramban asks that it doesn’t sat the commandment of birchas kohanim until Parshas Nasso?  The Rebbe (Likutay Sichos volume 22) explains that Rashi doesn’t mean that Aharon was fulfilling the commandment of birchat kohanim rather it was the text that Aharon chose to say his beracha.  Aharon used this text as a means of expressing a request for forgiveness for the agel. 

Based upon that explanation it is not problematic what the achronim ask why did only Aharon do birchat kohanim and not his sons, they also have the mitzvah?  Based upon this, the mitzvah wasn’t given yet, this was just Aharon’s personal blessing.