פירוש על שמות 9:6

.כל מקנה מצרים, all the cattle of Egypt. This refers to the cattle the Egyptians had not brought indoors, this is why the words "in the field" are missing here.

Adonoy will do this thing. The Torah explains that Hashem alone brought these two plagues (wild animals and pestilence), and not through the agency of Moshe and Aharon. Since these plagues came to enlighten them about Divine Providence, Hashem clarified that He has no need for intermediaries, for He Himself watches over the world and accomplishes everything.

וימת כל מקנה מצרים, “all the domesticated livestock of Egypt died.” The Torah means to emphasize that all the livestock which died belonged to the Egyptians. The verse does not mean that all the livestock in Egypt died, as it states specifically in our verse that of the livestock owned by Israelites not a single animal died. This is why the Torah could write in verse 20 that the livestock of G-d fearing Egyptians who had brought their beasts into shelter did not die.

וממקנה בני ישראל, and of the cattle of the Israelites none died. They owned cattle going back to the time Jacob had come to Egypt. On the other hand, they may have acquired cattle from the money they received from the Egyptians when they sold them water during the plague of blood. It is even possible that some smart Egyptians "sold" their cattle to the Jews before the onset of the plague [much as we sell chametz to the Gentiles before Passover Ed.] so as to escape the plague and not interfere with their cattle's grazing outdoors. The Torah then would tell us that such "sales" did not protect such cattle; only the cattle which were truly Jewish-owned survived the plague without having been indoors. We ought to understand what the verse tells us: 1) "All the cattle of the Egyptians as well as some of the cattle "owned" by the Jews died;" the Torah added the extra letter מ in the line וממקנה בני ישראל to tell us about these so-called Jewish-owned cattle which died. The letter מ applied only to part of the cattle and not to part of the sheep and goats. To sum up: animals owned outright by Jews did not die; animals owned by Jews in name only, died.