What are the implications of "Dei Hashiv lo"?
Rashi: It implies that he must redeem the entire field, and cannot redeem it in halves. 1
See Torah Temimah (citing Erchin, 30b), who learns this from "u'Matza K'dei Ge'ulaso" in Pasuk 26.
What are the implications of "Ad Sh'nas ha'Yovel"?
Rashi: It implies that Yovel releases the fields from the first moment - because it is Meshamet from the beginning of the year. 1
See also Sifsei Chachamim.
Why does a field revert to the seller in Yovel, but a house in a walled city does not? The Torah gave greater redemption rights for a house, i.e. he can redeem within one year!
Ramban, (29), Moshav Zekenim (29): It is difficult and shameful for one to sell his house, therefore he can redeem within one year. If he did not, he changed his residence and despaired; there is no harm if he cannot redeem afterwards. 1 A field is his source of income, which is indispensable, so it returns in Yovel.
Perhaps the Torah was also concerned lest people be loathe to buy houses if the seller could redeem it at any time. (PF)