For the source text click/tap here: Bechorot 37
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The Mishnah (37a) discusses a case in which a person sold meat under the presumption that it was permitted for consumption, but it subsequently became clear that it was prohibited:
"One who slaughters a firstborn animal, and it subsequently becomes known that he had not presented it for examination: What they have eaten, they have eaten, and he must return their money to them. As for what they have not eaten, the meat must be buried, and he must return their money.
Similarly, one who slaughters a cow and sells it, and it subsequently becomes known that it was a tereifah: What they have eaten, they have eaten; and what they have not eaten, they must return the meat to him, and he must return their money."
The Mishnah introduces a novel principle: even though the purchasers consumed the meat, they are not required to pay for it, and the seller must refund their money in full.
We explore the tension between economic benefit and religiously defined harm. The purchaser has received calories, nourishment, and perhaps considerable gastronomic pleasure. Yet the law may nevertheless treat the transaction as though no compensable benefit had been received.
This position invites us to distinguish pleasure from welfare, and welfare from holiness.