Matthew Poole and continuators on Exodus 29:26

26 And thou shalt take the breast of the ram of Aarons consecrations, and wave it for a wave-offering before the LORD, and it shall be thy part

To wit, the breast alone, whereas both shoulder and breast were given to Aaron afterwards: the reason whereof might be, either because Moses was not a proper and compleat Priest, as Aaron afterward was, but onely appointed by God for this time to do that work: Or because now there were in a manner two Priests, the one consecrating, to wit, Moses, the other consecrated, to wit, Araon, and therefore these parts were divided, the breast given to the former to be eaten, the shoulder offered unto God for the latter, verse 22. he being not yet a perfect Priest and therefore not in a capacity of eating it.

Marginal references: Lev. 8. 29