Above (3), he only mentioned buying what Na'ami sold. Why did he add buying from Rus, and establishing the Mes' name on his inheritance?
Rashi: You need to buy also from Rus, and she agrees only if you will marry her.
Malbim: You cannot redeem only Na'ami's part 1 . You can acquire Na'ami's part via a regular Kinyan. However, you can acquire Rus' only via Yibum. Via Yibum of Rus, automatically you acquire the field. It will be called on the name of the Mes' wife, and not on your name.
What is the reason? Tana'im argue about whether or not one who sold his field can redeem part (Kidushin 21a), i.e. Bal Korcho (against the will) of the current owner (the buyer), but if he consents, all would permit. Perhaps Na'ami [the current owner of her part] would object, for she wants a Go'el to marry Rus. There is more incentive for a Go'el if he will receive a full field!
