John Gill Commentary Numbers 8:13

"And thou shalt set the Levites before Aaron, and before his sons, and offer them for a wave-offering unto Jehovah." — Numbers 8:13 (ASV)

And you shall set the Levites before Aaron, and before his
sons
After atonement was made for them, as Aben Ezra observes; then they were presented to Aaron and his sons, to whom they were given to be their ministers, and assistants to them:

and offer them [for] an offering to the Lord :
or, and "waved them", &c. as before, that is, order them to be waved; for not Moses, but Aaron, did this, and indeed was done already; and therefore should rather be rendered, as by Bishop Patrick, after you have offered, or waved them.