Deuteronomy 23:24-25 says "If you enter your neighbor's vineyard, you may eat all the grapes you want, but do not put any in your basket. If you enter your neighbor's grainfield, you may pick kernels with your hands, but you must not put a sickle to his standing grain."
Deuteronomy 24:19-22 says "When you are harvesting in your field and you overlook a sheaf, do not go back to get it. Leave it for the alien, the fatherless and the widow, so that the LORD your God may bless you in all the work of your hands. When you beat the olives from your trees, do not go over the branches a second time. Leave what remains for the alien, the fatherless and the widow. When you harvest the grapes in your vineyard, do not go over the vines again. Leave what remains for the alien, the fatherless and the widow. Remember that you were slaves in Egypt. That is why I command you to do this."
When I lived in Aurora, the apartment complex was at the edge of the city, next to a farmer's field. If the inhabitants of our complex had tried this kind of "snack harvest" we'd have taken out about a full row's worth of grain: two if the folks from the complex a few blocks down had joined in. Maybe the farmer could stand the loss, but it wouldn't be trivial.
So set aside the literal application of this. Never mind about trying to find modern equivalents (the grocer survives on a thin enough profit margin as it is). What is the principle to follow, or the attitude we're to cultivate?
I think both passages are really aimed at the owner. The first asserts a limited claim of one neighbor on the produce of another. One principle is that the owner is not entitled to demand the entire produce of his land. The second passage asserts the same principle, though in a slightly different context. The first passage says that neighbors have a share, and the second that the helpless have a share. The owner of the land is not the absolute owner.
We are accustomed to seeing this principle in a different context: property taxes. The owner is not the absolute owner because the state has a permanent lien on his land for taxes.
In this case in Deuteronomy the limit on ownership is not so easily calculated or predicted. Neighbors may come around frequently, or there may be more poor around than usual (Ruth 2:15-16). You can think of this limit as creating a certain fuzziness about what is mine and what my neighbor has a claim on. My neighbor is not entitled to steal: going into my vineyard with a basket goes beyond neighborly commonality into greed and theft. But I am not entitled to demand every kernel of corn from my field.
Interestingly enough, if I plant to the edges of my land and harvest every grain, I leave no room and no food for the overlooked birds and insects that pollinate and help keep my land fertile and versatile. If I plant every year and don't let the land lie fallow now and then I'll have to drag in extra fertilizers. Once again, if I assert absolute ownership I run into troubles--not with God's law this time (at least not explicitly) but with ecological law.
Can this sort of fuzziness ever be made part of statutory or common law? I can recall instances when this would have been a useful principle. Recall George Lucas and the "Phantom Edit" revisions. However amused Lucas may have been by the revisions, he had no choice but to assert his property rights over the material, on pain of losing it all. There was no room for fuzziness--and there ought to have been some. Changing the law would have all sorts of unintended consequences, and I'm not advocating such changes; merely showing that justice and art would have been better served with less claim to absolute ownership in this case.
At any rate, the intent of the Deuteronomy passages seems clear enough: I am not to consider myself as absolute owner, and am to consider my neighbor and the helpless as having liens upon my increase/income. This falls short of the New Testament view, but it is hard enough to obey the Deuteronomic rule.