Commentary for Avodah Zarah 31:12
וכי משהי ליה בריא ועביד מלאכה אמר רב אשי אמר לי זבידא בר תורא משהינן ליה ועביד על חד תרין:
But can [a fatted ox] be kept [and slimmed] and made healthy enough to do work? Ashi said: Zavida told me that a young bullock [that was fattened] they can hold on to it [and slim it] and it will do the work of two.
Daf Shevui to Avodah Zarah
Ashi affirms that if a fatted ox is kept and slimmed down it can eventually perform the work of two. This is why it is problematic to sell them a fatted ox.
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