Bechorot 10 - If The Firstborn Donkey Is Not Redeemed...
If one does not wish to redeem his firstborn donkey, he must decapitate it with a butcher's cleaver, from the back of the neck. Rabbah said: "Even Rabbi Shimon, who permits using the firstborn donkey before redeeming it with a sheep, agrees that after decapitation it can't be used. What is the reason? Compare it to a calf that was decapitated to atone for an unsolved murder case; this calf was buried in the valley and not used. So too the decapitated donkey cannot be used."
How does Rabbah know Rabbi Shimon's opinion? Because in the list of foods that can become impure Rabbi Shimon does not include the meat of a decapitated donkey. We must conclude that Rabbi Shimon deems it forbidden for benefit, thus, one cannot sell it even for others to eat. However, the Talmud rejects his proof: perhaps donkey meat is not food simply because people don't usually eat it? This attack is parried: the prohibition of the Torah to eat it elevates it to the status of food.
And Rabbah? "Since he caused loss to a kohen, his donkey should now be lost to him" describes the position of Rabbi Shimon.
Art: Ebenezer Newman Downard - A Donkey At A Gate