כִּ֤י יַבְעֶר־אִישׁ֙ שָׂדֶ֣ה אוֹ־כֶ֔רֶם וְשִׁלַּח֙ אֶת־בעירה [בְּעִיר֔וֹ] וּבִעֵ֖ר בִּשְׂדֵ֣ה אַחֵ֑ר מֵיטַ֥ב שָׂדֵ֛הוּ וּמֵיטַ֥ב כַּרְמ֖וֹ יְשַׁלֵּֽם׃ (ס)
Se um homem causar um incêndio no campo ou na vinha de outro, e enviar seus animais para pastar ali e queimarem o campo de outro homem, ele fará restituição com o melhor de seu próprio campo e o melhor de sua própria vinha. {ס}
Tosefta Bava Kamma
Anything that I am obligated in its guards, I am obligated in its damage (B.K. 1:2)--that is the ox and the pit. [If] I have rendered it fit to cause damage, even in part, it is like rendering it fit to cause damage, in full--that is the pit. [If] I am liable to make restitution for its damage--it teaches that that the owner [of the injured animal] attends to (i.e., maintains ownership of) the animal carcass. Property that does not have "sacrilege" (i.e., that is consecrated to the Temple)--it excludes an ox of an Israelite that gored an ox of the Temple, and an ox of the Temple that gored an ox of an Israelite. Property that does not belong to the children of the covenant--it excludes an ox of an Israelite that gored the ox of a Canaanite. Private property--it excludes ownerless property.
Tosefta Ketubot
Why did they say that damages are evaluated from the best property? Because of robbers and violent men, such that each should say, "Why would I steal? Why would I be violent? For tomorrow the court will evaluate my best field!" They relied on the verse "The best of his field, the best of his vineyard he should pay" (Shemot 22:4).