For the source text click/tap here: Bava Kamma 66
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A change to the stolen object effects its acquisition by a thief. He is still liable for his theft, but if he kills and sells an ox or sheep, he is not liable for the fourfold or fivefold payment because now he slaughters his own animal.
In the phrase "...and he shall return the stolen article that he stole..." the extra words "as he stole" teach that if the article is as it was when he stole, he returns the article, but if the thief transformed it, he is not required to return the stolen article itself, but rather its value.
The rabbis debate whether or not ye'ush makes sense.
What if matzah was stolen but peach elapses by the time that the thief has been found? Why wouldn't the thief say that he will simply return the matzah? Why would the thief consider the matzah, now useless, to have been acquired?
The rabbis consider the thief's opinion and the opinion of the person who despairs the loss of the matzah. They determine that if the victim does not want their item returned, as it is useless, then the robber should pay the value of the matzah even if he no longer wishes to acquire it.
We explore the possible scenarios of the more fascinating phenomena in halakha in which a mitzva and aveira intersect.