Having stated "Vechishav im Koneihu", why does the Torah see fit to add "Im Od Rabos ba'Shanim ... " (and "v'Im Me'at Nish'ar ba'Shanim" in the following Pasuk)?
Rashi: The Torah is simply clarifying its previous statement 1 - that the accurate assessment that one makes needs to be based on the remaining years until the Yovel.
Seforno: To teach us that one deducts from the total the number of years that the Eved Ivri worked, 2 irrespective of the fact that if many years remain until the Yovel, he would have become more experienced and produced better work, and that, if few years remain, the work that he did was not on a par with the work that he would have performed had he remained until the Yovel.
Moshav Zekenim (from Kidushin 20a,b): Our verse discusses if his value increased (he was sold for 100, and his value rose to 200), the redemption is based on Kesef Miknaso (100 for all the years of his sale). The next verse discusses if his value decreased (from 200 to 100). The redemption is "k'Fi Shanav" based on his current value (as if he were sold for 100). We are always lenient to minimize the redemption.
What do we learn from "l'Fihen Yashiv Ge'ulaso mi'Kesef Miknaso"?
Moshav Zekenim (from Kidushin 20a,b): If his value increased, the redemption is based on Kesef Miknaso. We considered what he was sold for, and deduct for the years that he served. We ignore his current value.