Maasrot, Chapter Two, Mishnah Six
This mishnah teaches ways in which a person can buy produce and yet continue to eat it without tithing.
1) One who says to his friend: Here is an issar for ten figs which I choose for myself, he may choose them and eat [one at a time without tithing].
a) [If he said] For a cluster of grapes which I choose for myself, he may pick grapes from the cluster and eat.
b) [If he said], For a pomegranate which I choose for myself, he may take apart [the pomegranate] and eat [it one piece at a time].
c) [If he said] For a watermelon, which I choose for myself, he may slice and eat [it one piece at a time].
2) But if he said For these twenty figs, or For these two clusters, or For these two water-melons, he may eat them in his usual way and be exempt [from tithe], because he bought them while they were still attached to the ground.
Section one: In all of the cases in this mishnah, the person gives the person money on condition that he chooses which particular piece of fruit he is buying. The fruit is still attached to the ground. The purchase is only completed once he picks the fruit. Therefore, he can eat them without tithing, but only one at a time, as was Rabbi Judahs opinion in yesterdays mishnah. The same is true for the grapes, pomegranate and watermelon mentioned in the remainder of this section. Since he doesnt determine which piece of fruit he is buying until he picks it, this is considered a case of buying produce that is already detached from the ground.
Section two: In this case he specifies which of the fruit he wants to eat. In this way, he succeeds in acquiring the fruit while it is still attached to the ground. Buying fruit while it is still attached to the ground does not make it liable for tithes, and therefore, he can eat from it in a chance fashion before it is tithed.