Matanot Aniyim

When a person harvests his field, he should not harvest the entire field. Instead, he should leave a small portion of the standing grain at the end of his field, as [Leviticus 23:22] states: "Do not completely remove [the grain in] the corners of your field when reaping." [This prohibition applies] to one who reaps and one who uproots. [The grain] left [standing] is referred to as pe'ah. Just as one leaves [pe'ah] in his field, so too, [he must leave pe'ah] for trees. When he gathers his produce, he should leave some for the poor. If he transgressed and harvested the entire field or gathered all of the produce of the trees, he should take some of what was harvested or gathered and give it to the poor.
Giving [this produce] fulfills a positive commandment, as it is stated [ibid.]: "Leave it for the poor and the stranger." Even if one ground the flour, kneaded it, and baked it into bread, he should give pe'ah from it for the poor. If the entire harvest that was reaped was destroyed or consumed by fire before he gave pe'ah, he is liable for lashes. [The reason is that] he violated a negative commandment and he did not fulfill the positive commandment that could correct it. Similarly, with regard to leket: When one harvests or binds sheaves, he should not gather the stalks that fall during the harvest. Instead, he should leave them for the poor, as it is stated [ibid.]: "You shall not gather the gleanings of your harvest." If he transgresses and gathers them - even if he ground them [into flour] and baked [them], he must give it to the poor, as it states [ibid.]: "Leave it for the poor and the stranger." If [this produce] is lost or consumed by fire after he gathered it, but before he gave it to the poor, he is liable for lashes. Similar [laws apply to] individual grapes that fall during the grape harvest and to underdeveloped grape clusters, as it is stated [ibid. 19:10]: "Do not harvest underdeveloped grape clusters from your vineyard, nor gather individual grapes that fall in your vineyard. Leave it for the poor and the stranger."
Similarly, if a person is binding sheaves of wheat into bundles and forgets one bundle, he may not go back and take it, as [Deuteronomy 24:19] states: "If you forget a sheave in the field, do not return to take it." If he transgressed and gathered it - even if he ground it [into flour] and baked [it], he must give it to the poor, as it states [ibid.]: "They shall be for the stranger, the orphan, and the widow." This is a positive commandment.
Thus you have learned that they all are prohibitions that can be corrected by positive commandments. If one [transgresses and] does not fulfill the positive commandment involved, he is worthy of lashes. Just as [the prohibition against taking] forgotten produce (shichichah) applies with regard to sheaves, so too, it applies to standing grain. If one forgot standing grain and did not harvest it, it should be [given] to the poor. Just as [the prohibition against taking] forgotten produce applies with regard to grain and the like, so too, it applies to all [fruit-bearing] trees, as it is stated [ibid.:20]: "When you beat your olive tree, do not go back and take its glory." This law also applies to [produce from] other trees. Thus it can be concluded that there are four types of presents given to the poor in a vineyard: individual grapes that fall, underdeveloped grape clusters, pe'ah, and forgotten produce. There are three presents from a grain crop: leket, forgotten produce, and pe'ah, and two from trees: forgotten produce and pe'ah. The owners do not have the right to give these presents to the poor to the individual of their choice. Instead, the poor may come and take it against the owners' will. [These presents] are expropriated even from a poor Israelite.

There is a sixth present [awarded] to the poor from the land's produce: the tithe given to the poor. It is called the tithe for the poor. This is the order of [the separation of] the terumot and the tithes. After one harvests produce from the earth or fruit from the tree and completes all the necessary work, he separates one fiftieth of the produce. This is called the great terumah and should be given to the priest. Concerning this the Torah states [Deuteronomy 18:4]: "The first of your grain, your wine, and your oil." Afterwards, he separates one tenth from the remainder. This is called the first tithe and must be given to the Levite. Concerning this, the Torah states [Numbers 18:24]: "For the tithes of the children of Israel..." and [ibid.:24] states: "To the descendants of Levi have I given all the tithes within Israel." Afterwards, he separates one tenth from the remainder. This is called "the second tithe." This belongs to the owners and is eaten in Jerusalem. Concerning this, it is stated [Leviticus 27:31]: "If a person shall redeem his tithes and [Deuteronomy 14:22-23] states: "You shall surely tithe.... And you shall partake of it before God your Lord in the place that He will choose." These portions should be separated in this order in the first, second, fourth, and fifth years of the Sabbatical cycle. In the third and sixth years of that cycle, by contrast, after the first tithe is separated, a different tithe is separated from the remainder and it is given to the poor. It is called the tithe for the poor. In these two years, the second tithe is not separated, only the tithe for the poor. Concerning it is written [Deuteronomy 14:28-29]: "At the conclusion of three years, you shall bring out the tithe of all your grain in that year and deposit it in your gates and the Levite... [and the stranger, the orphan...] shall come." And concerning it, it is written [ibid. 26:12]: "When you will complete your tithing [in the third year]...."