Demai, Chapter Three, Mishnah Two
This mishnah teaches that in certain cases a person must tithe produce even if he is getting rid of that produce or it is not yet even his.
1) One who wants to cut off leaves of vegetables in order to lighten his burden, he may not throw them down until he has [first] tithed them.
2) One who takes vegetables from the market [with the intention of buying them], and then decides to put them back, he should not put them back until he has [first] tithed them, for nothing is missing [after they are tithed] except for their quantity.
3) But if he was standing [there and deciding what to] buy and then saw another load of better quality, he may put them back [untithed], since he had not yet drawn them into his possession.
Section one: A person while traveling might discard leaves of vegetables because he doesnt intend to eat them. Discarding them will lighten his load. The problem is that a poor am haaretz might come by and pick up the leaves and eat them because he has nothing else to eat. To prevent the am haaretz from unwittingly eating untithed produce he should tithe the leaves before he discards them. One should always be cautious that his actions dont cause others to stumble.
Section two: There are two conflicting interpretations of this section. According to Albeck (according to whose interpretation I translated), when the person in our mishnah picks up the vegetables he has not yet bought them, but is only considering buying them. When he changes his mind and decides not to buy them, he shouldnt return them to the seller until he tithes them. The mishnah explains that tithing doesnt cause a loss to the seller because the seller, or the one who later on buys from him, is going to have to tithe in any case. This is the meaning of nothing is missing except for their quantitythe value has not gone down. Although this is the rule, a person should not intentionally put himself in such a situation because the seller probably doesnt want others tithing his produce.
According to the Bavli and the commentators who explain accordingly, the purchaser has acquired the vegetables by picking them up even though he hasnt counted them yet. This is the meaning of nothing is missing except [counting] the quantity. Since they belong to him, he cant return them until he tithes them.
Section three: In this section the person has not yet picked up the vegetables. He can put them back, meaning he can change his mind, since they have not yet become his. When he puts them back he need not tithe them.