A change to the stolen object effects its acquisition for 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.