Given that "Ki Yav'er", "Be'iro" and "u'Bi'er" are all Leshonos of 'Beheimah (as in Bamidbar 20:4), what is the meaning of the opening phrase in the Pasuk?
Rashi: "He sends his animal into his friend's field or vineyard".
Rashbam: He feeds his animal in his friend's field.
Seforno: It means that he keeps his animal in his own field - and it strayed on its own into his neighbor's property.
Targum Onkelos: "He damages (treats like Hefker) his friend's field or vineyard.
What kind of damage is the Pasuk referring to?
Rashi: "Regel" - the animal cases damage by trampling over the Nizak's property; "Shein" - it eats his crops. 1
What are the ramifications of "Meitzv Sadeihu ... Yeshalem"?
Rashbam #1: If it is not sure exactly what his animal ate, he must pay according to the Nizak's best field, in case that is what it ate. 4
He does however, have the option of paying money or Metaltelim - even bran (See Sifsei Chachamim).
The amount of the damage and no more.
This Din extends to all forms of Nezikin..
This is problematic however, since, in case of doubt, one ought to apply the principle 'ha'Motzi me'Chavero, alav ha'Re'ayah'?
Why are damages paid only from Idis (highest quality land)?
Bava Kama (7b): So it will be easy for the victim to sell it.
Riva #1: It is because a person prefers his own to another's (if the victim receives the same quality, he was not properly compensated).
Riva #2: His own property is close to him (more convenient. If he receives the same quality, he was not properly compensated.)
Riva #3: It is a deterrent, so people will not damage. 1
Riva: A borrower did not intend to damage, just he cannot pay. He should pay Ziburis (lowest quality land), just Chachamim enacted Beinonis, lest people be loathe to lend. A Kesuvah is collected from Ziburis, for she did not lose anything.