Sforno on Exodus 34:19/ספורנו על שמות ל״ד:י״ט

1כל פטר רחם לי, the commandment applies to male humans and male new born animals of the categories that are domesticated and fit to eat for Jews. Seeing that one category of “unclean” animal, the donkey, is also part of this legislation, though, of course such an animal is not fit to be sacrificed on the altar of the Temple, the Torah has to add certain qualifications when writing about this commandment. Whereas the “pure” firstborn may not be redeemed and made secular, but must themselves be offered on the altar, their blood and fat remaining there, the donkey either has to be redeemed or to be killed in a manner that precludes its flesh being used commercially. [it may not be consumed seeing it is an “unclean” animal. Ed.] (Numbers 18,17)