וכי תמכרו ממכר לעמיתך או קנה מיד עמיתך אל תונו איש את אחיו
And if you sell something to your neighbor, or buy something from your neighbor's hand, you shall not defraud one another (Leviticus 25:14)
Rashi explains that this refers to monetary oppression. Below (Leviticus 25:17) it is written: "You shall not wrong one another" (ולא תונו איש את עמיתו), and Rashi explains:
Here the Scripture warns about verbal oppression, and the Scripture therefore writes: "and you shall fear your G-d" (ויראת מאלהיך) because He knows that which is given over to the heart.
And it is a wonder that on this verse: "and if you sell something to your neighbor or buy from your neighbor" (וכי תמכרו ממכר לעמיתך או קנה מיד עמיתך אל תונו איש את אחיו), the Midrash says (Leviticus Rabbah 33:1):
This bears on the text (Proverbs 18:21) "Death and life are in the power of the tongue" (מות וחיים ביד לשון וכו').
Thus, the Midrash, unlike Rashi, understands the first verse to refer to verbal oppression. However, this interpretation is untenable, because the Scripture explicitly refers in this verse to an act of purchase or of sale.
But it appears to our master that the Midrash meant to explain why the Scripture writes "וכי תמכרו ממכר" (and if you sell something that is sold) when it would have been more appropriate to write "וכי תמכרו דבר או חפץ" (and if you sell a thing or an object) inasmuch as the word "ממכר" applies only after the object has already been sold and not before. See the interpretative rules of the Malbim. One could also ask why the Scripture did not write the conjugated form "תקנו" (you will buy) parallel to "תמכרו" rather than the root form "קנה" (buy) (i.e., וכי תמכרו ממכר לעמיתך או תקנו מיד עמיתך). The Midrash therefore explains that with this golden expression the Torah wanted to inform us of a great precept, which is that if the buyer knows that the seller is selling because he is in financial distress and must sell his belongings, the buyer should not take advantage of the seller's distress and pay him less than the usual value of the object. Similarly, if the seller knows that the buyer very urgently wants his merchandise, he should not raise the price unduly.
Now if the seller is forced by circumstances to sell his belongings, then the belongings may be considered as if they had already been sold to anyone willing to pay even a nominal price for them. (And see the words of our master about this in poroshat Mishpatim on כי תקנה עבד עברי.) That is why the Scripture says "if you sell something that is sold" referring to the object sold as "ממכר," i.e., something that has already been sold because of the distress of the seller. Similarly, the Scripture writes "או קנה מיד עמיתך" (or buy from the hand of your neighbor) using the root form to show the urgency of the purchase, because the buyer is willing to buy even before he reaches a fair bargain with the seller and is willing to pay whatever price he is asked by the seller. It is concerning such a situation that the Scripture warns "do not wrong each other" (אל תונו איש את אחיו), do not conduct yourself guilefully with your neighbor.
Now the Talmud discusses (Bava Kamma 7a) one who has houses, fields and vineyards, but cannot sell them. The Talmud asks how it is possible that he is not able to sell them, and the answer given is that since the seller is desperate for cash, the price of his property is depressed. This indicates that by divulging his financial distress to others, he has caused the value of his property to fall. So it is to such a situation that the Midrash is referring when it says that life and death are under the control of the tongue, for if a person is careful to guard his tongue and not to divulge his financial distress to anyone, what he is trying to sell would not be considered to have already been sold before he sold it.
והארץ לא תמכר לצמיתות כי לי הארץ כי גרים ותושבים אתם עמדי ובכל ארץ אחוזתכם גאולה תתנו לארץ
The land shall not be sold forever; for the land is mine; for you are strangers and sojourners with me. (Leviticus 25:23)
And it may be questioned why this verse appears here. Does this entire chapter not speak about the sabbatical year for the land? And our master says that this verse also suggests the law of the sabbatical year, by telling us that the land may not be sold in perpetuity so that it would be tilled constantly. Rather, all the land belongs to the Eternal, and you are just strangers dwelling upon it. You are merely sojourners for as long as I desire it, and you may work the land for only six years, but on the seventh year, the land will enjoy its sabbath.