If, as the Chachamim have taught (in Bava Metzi'a, 56a), Karka (land) is not subject to Ona'ah, how will we explain the current Pesukim?
Ramban #1: Pesukim 14 - 16 are speaking (not about Ona'ah, 1 but) about the Din of Yovel - inasmuch as a field can only be sold according to the number of anticipated produces until the Yovel, when it must be returned, come what may.
Ramban #2: The current Pesukim are referring to Karka, too. This is because, even though Karka is not subject to Ona'ah, it is nevertheless subject to the Isur of Lo Sonu. 2
Seforno: Based on the Gemara in Bava Metzi'a that Ona'ah of even a fraction applies to anything that is measured, weighed or counted, falsifying the number of years - that are counted - is subject to Ona'ah.
Why does the Torah reckon the years "after the Yovel" and not from the Yovel itself?
Seforno: Because it is unwise to purchase a field in the Yovel itself, when the owner may return and retrieve it before the purchaser has derived any benefit from it .
What are the connotations of "be'Mispar Sh'nei Tevu'os Yimkor lach"?
Rashi #2 and Ramban #2 (citing Erchin, 29b): The Pasuk is teaching us that the owner is not permitted to redeem his field from the purchaser before two years - even if the field yields three years' produce during that time.
Seforno: It implies that the sale only covers the years of harvest, but not those of drought or of withering. 3
This is also how the Rashbam explains the words "Ki Mispar Tevu'os ... " in the following Pasuk.
Rashi: And that consequently, if the seller charges a higher price than the few years leading to the Yovel, the seller is guilty of Ona'ah, whereas if the purchaser offers to pay a lower price than the many years until the Yovel, then he has transgressed.
Seforno: As is the Din with regard to someone who rents a field
Bearing in mind the fact that Karka is not subject to Ona'ah, as the Gemara in Bava Metzi'a, 56a, learns from Pasuk 14, how will we explain the current Pesukim?