Why the double Lashon "Im Himatzei Simatzei"?
Targum Yonasan: 'If witnessess testify that he has the ox ... in his possession'.
Bava Kama, 75a): To teach us that if the Ganav admits to have stolen the article, he is Patur from paying - even if witnesses subsequently arrive in Beis-Din. 1
'Modeh bi'Kenas v'Achar-Kach Ba'u Eidim, Patur' (See Torah Temimah, note 21).
Why does the Torah insert the word "be'Yado"?
Rashi and Rashbam: To teach us that if the Ganav still has the animal in his possession (and has not Shechted or sold it), he pays (not four or five times, but) double. 1
Why is a Ganav Chayav Kefel for stealing a donkey, but nor fourfold for Shechting or selling it?
Oznayim la'Torah: Based on the principle 'Shechitah she'Einah Re'uyah Lo Sh'mah Shechitah', non-Kasher animals are not subject to the Din of Arba'ah va'Chamishah for Shechting them, and consequeently, not for selling them either, 1 whereas there is no reason to preclude them from the Din of Kefel.
Since Mechirah is compared to Shechitah.
What is the significance of the word "Chayim"?
Rashi: To teach us that the Kefel must consist of a live animal (or its value) and not of a dead one. 1
Rashbam and Hadar Zekenim: It implies tht he did not Shecht it
Even though Arba va'Chamishah is based on the value of the ox at the time that the Din Torah takes place (See Torah Temimah, 29).