Or HaChaim on Numbers 19:5/אור החיים על במדבר י״ט:ה׳

1 ושרף את הפרה לעיניו, את ערה, "He shall burn the heifer in his sight; its skin, etc." Why does the Torah interrupt the sequence of what is to be burned by mentioning לעיניו, "in his sight?" We can understand this in light of a ruling by Maimonides in chapter 4 of Hilchot Parah Adumah that the red heifer should be burned whole, but that in the event the priest first skinned the heifer and cut it up before he burned it this was acceptable. The reason the Torah interposed the word לעיניו where it did was to alert us to this alternative way of burning the red heifer. The Torah wrote ושרף את הפרה to indicate that the entire heifer had to be burned at the same time even if it had already been cut up. The words את ערה, "with its skin," indicate that it all has to be burned together though it need not necessarily be when the heifer is still a whole cadaver.

2על פרשה ישרף, "with its dung it shall be burned." The reason the Torah repeats the apparently superfluous word ישרף, "he shall burn it," is to show that there are two ways in which this burning could take place, as I have explained. Either one burns the heifer while it is whole, or after it has been skinned and cut up. The second word ישרף refers to the second alternative. Maimonides in Hilchot Parah Adumah chapter 4 rules (based on a Tossephta in the second chapter of Parah) that if some part of the skin fell off, or even some of its hair in an amount equal in size to that of an olive, one has to put it back so it can be burned with the animal or the whole procedure becomes invalid. In other words, the second word ישרף may tell us that it is indispensable that all the parts of the heifer be burned.