Two very similar phrases occur in Genesis 27.
Genesis 27:28 (Isaac speaking to Jacob):
וְיִתֶּן־לְךָ הָאֱלֹהִים מִטַּל הַשָּׁמַיִם וּמִשְׁמַנֵּי הָאָרֶץ
veyitten-leka ha-elohim mittal ha-shamayim umishmannei ha-aretz
Genesis 27:39 (Isaac speaking to Esau):
מִשְׁמַנֵּי הָאָרֶץ יִהְיֶה מוֹשָׁבֶךָ וּמִטַּל הַשָּׁמַיִם מֵעָל
mishmannei ha-aretz yihyeh moshaveka umittal ha-shamayim meʿal
Basically, the crux of the issue is how the prefixed מ functions in each verse.
In Genesis 27:28, Isaac clearly blesses Jacob, as indicated by the final clause in the verse (“plenty of corn and wine”):
28 Therefore, God shall give you of the dew of heaven, and the fatness of the earth, and plenty of corn and wine.
Here, then, the מ in conjunction with a conjugation of the verb נָתַן (“to give”), is functioning partitively (i.e., “to give part/some of”)
God will give Isaac some (part) of the dew of heaven and fatness of the earth, thus providing him with an abundance of corn and wine.
However, in Genesis 27:39, the prefixed מ is being used locatively, to describe one’s location, or in this case, one’s dwelling, away from some other location (or thing). Gesenius notes that this usage can have the figurative meaning of “without” (i.e., privative).
Instead of being used with the verb נָתַן (natan), it is being used with יִהְיֶה (yiyeh), a conjugation of הָיָה (hayah), “to be.” These two verbs dramatically change the function of מ in each verse.
Jacob is blessed; Esau is cursed.3 Note Carl Keil’s commentary on this verse:4
“Behold,” so reads his saying, “your dwelling shall be without the fat fields of the earth and without the dew of heaven from above.” By a play-on-words, Isaac uses the same expression, מִשְׁמַנֵּי הָאָרֶץ [“from the fatness of the earth”] and מִטַּל הַשָּׁמַיִם [“from the dew of heaven”], as v. 28, but in the opposite meaning, מִן there in a partitive [sense], here in a privative sense: away from, without. The context requires the words to be comprehended thus, and not with the Vulgate, Luther, et al.: “your dwelling shall be of the fat of the earth and the dew of heaven.”
Since Isaac (v. 37) has allocated to Jacob the blessing of the super-abundance of corn and wine, he can’t possibly afterwards promise Esau the fat fields and the dew of heaven. Nor would these even suit the following words at all, “You shall live by your sword.” The privative sense of מִן, by the way, is genuinely poetical (cf. 2 Sam. 1:22; Job 11:15, etc.). Therefore, the saying contains the thought [that] Esau’s dwelling would be the opposite of the blessed land of Canaan; it would be an unfruitful land.
1 Gesenius, p. 481, מן (min), 1
2 id., p. 483, מן (min), 3, b
3 Well, I guess that is all relative. Living in such lands, away from bountiful resources, Esau will develop the ability to survive in harsh conditions, to become resourceful. On the other hand, living in bountiful lands, Isaac’s land will become the envy of many people, thus causing him to often defend it from those wishing to take it for theirselves.
4 Keil, p. 278
Gesenius, Heinrich Friedrich Wilhelm. Gesenius’s Hebrew and Chaldee Lexicon to the Old Testament Scriptures. Trans. Tregelles, Samuel Prideaux. London: Bagster, 1846.
Keil, Carl Friedrich. Commentary on the Old Testament. 1900. Reprint. Trans. Martin, James. Vol. 1. Grand Rapids: Eerdmans, 1986.