Maaser Sheni, Chapter 5, Mishnah 5. The Mishnah continues to discuss how kerem reva'i is redeemed, and adds two special cases: redemption during the Shemittah year, and redemption of produce that was declared ownerless.
Two special cases of redemption:
"Uva'shevi'is - podehu be'shavyo" - Maaser Sheni does not apply in the Shemittah year, since Shemittah produce is not subject to Terumah and maasros; but the law of kerem reva'i does not depend on Terumah and maaser in and of itself, and therefore it applies even in Shemittah. A vine that reaches its fourth year during the Shemittah year is governed by the laws of kerem reva'i. Its redemption is made according to its plain market value, without deducting expenses, because in Shemittah one does not hire workers for field labor, and even the gathering is done by individuals and involves no payment. Since there are no expenses at all, there is nothing to deduct from the value.
"Ve'im hayah ha'kol mufkar - ein lo ela s'char lekitah" - In any year other than Shemittah, produce that was declared ownerless resembles Shemittah produce somewhat, but not entirely: the one redeeming deducts from the value only the wages of gathering. For an ownerless item has no one tending to it, and it involves no additional expenses; only the cost of the harvesting itself exists, and therefore that amount is deducted from the value.
"Ha'podeh neta reva'i shelo - mosif alav chumsho":
This ruling follows Beis Hillel. Beis Shammai and Beis Hillel disagreed as to whether the added fifth applies to kerem reva'i, and Beis Hillel held that it does. Our Mishnah is taught anonymously, and it adopts the position of Beis Hillel. And of course, the fifth applies only to one's own neta reva'i - just as with Maaser Sheni, where a fifth is added only when a person redeems his own produce, and not that of another.
"Bein she'hu shelo u'vein she'nitnah lo be'matanah":
There are two cases here: whether he grew it himself, or whether it was given to him as a gift. The Rambam points out here that this cannot refer to an ordinary gift, since kerem reva'i has the status of mamon gavoha, property belonging to Heaven - like Maaser Sheni - and it cannot be given away as a gift. Rather, the case is where it was given to him before it reached onas hamaasros, before a third of its growth, at which point it does not yet have the status of mamon gavoha, and it can be given to him as a gift.
In summary: In this Mishnah we learned that kerem reva'i applies even during the Shemittah year, and then it is redeemed at its full market value, since in Shemittah there are no expenses for labor and gathering; produce declared ownerless in other years is redeemed with only the gathering wages deducted; and one who redeems his own neta reva'i adds a fifth, following the view of Beis Hillel - both regarding what he grew himself and what was given to him as a gift before onas hamaasros, when it did not yet have the status of mamon gavoha.