Command Aharon and his sons. (6:2)
Rashi quotes Toras Kohanim that comments, “Tzav, the word, ‘command,’ can only be meant to express urging on, miyad u’le’doros, for the immediate moment and for future generations. (Furthermore) The Torah must especially urge in a situation where there is a loss of money.” The Kohanim sustain a financial loss because they are not paid for their service. In order to perform it, they must be readily available, thus precluding any other form of livelihood. The Torah makes this point within the context of the Olah – Elevation/Burnt Offering, because the loss of income in this case is especially significant….
