This saying is difficult to interpret. Its thrust seems to be that no matter how much the land of poor people produces, they suffer from the social and economic injustices done to them and thereby lose what they have. As Revised Standard Version expresses it, the saying is a condemnation of those who take from the poor.
“The fallow ground of the poor yields much food”: This line is literally “Plenty of food [is] fallow ground the poor.” “Fallow ground” refers to ground or land that is left unseeded for a growing season. It may or may not be tilled, plowed, cultivated. Some object that such ground as is mentioned here cannot yield much food. Fallow ground is left unseeded to restore it. The Hebrew term may refer to land left without planting for a season or it may refer to idle land, that is, land that is not being used or land that is newly claimed and has not grown crops before.
Translations differ greatly in their handling of this saying. Some speak of a potential or conditional situation; for example, “Even when the land of the poor produces good crops. . .” (Contemporary English Version), and “The field of the poor may yield much food” (New Revised Standard Version). See Good News Translation “could yield.” Others make a statement of fact, for example, Biblia Dios Habla Hoy “In the field of the poor there is abundant food.”
“But it is swept away through injustice” is literally “but substance [is] swept away without justice.” There is much guesswork in translating line 2. Revised Standard Version “it”, referring back to “food” in line 1, does not reflect anything in the Hebrew text; the sense of “substance” is unclear here, although the term clearly has a meaning of “wealth” (Revised Standard Version) or “riches” in 8.21. “Swept away” suggests being taken away or captured, and, in the context, to being lost or being cheated out of it by others. According to Good News Translation it is the “unused fields” that “unjust people” will not let the poor use. Contemporary English Version has the poor getting “cheated out of what they grow,” that is, their food. “Injustice” refers to the failure of people to act fairly, in a just or right manner. In light of the difficulties of this text either Good News Translation or Contemporary English Version may be used as translation models.
Quoted with permission from Reyburn, William D. and Fry, Euan McG. A Handbook on Proverbs. (UBS Helps for Translators). New York: UBS, 2000. For this and other handbooks for translators see here .

The Good News translation makes the most sense.