Maaser Sheni, Chapter Five, Mishnah Five
During the sabbatical year, it is forbidden to work the land. Therefore, there is no cost in growing the produce, and one who redeems it cannot reduce this cost.
1) But in the seventh year he must redeem it for its full value.
2) If [in other years] it had all been made ownerless property, the person [who redeems it] can only claim the cost of picking it.
3) One who redeems his own plant in its fourth year, he must add a fifth of its value, whether the fruit was his own or was given him as a gift.
Section one: As was explained in the introduction, since one cannot work the land during the sabbatical year, when redeeming it there are no costs to reduce from the produce. It must, therefore, be redeemed for full market value.
Section two: If in other years during the sabbatical cycle, one makes his field ownerless, and then someone comes and picks the produce, the one who redeems the maaser sheni can only claim the costs involved in picking it. These will be quite minimal, just what one would pay to have someone pick his produce.
Section three: This follows the opinion of Bet Hillel from mishnah three, who holds that the same rules that govern maaser sheni govern fourth year produce, and it is the same rule we saw in 4:3 with regard to maaser sheni.