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 the Torah a field to revert to its original owner in the Yovel?
Ramban (on Pasuk 29): because it is his source of Parnasah and is therefore indispensable.