Gemara

66b - Question: Regarding a beam, Shinuy ha'Shem (to 'roof') does not acquire!

(Mishnah): If one built a stolen beam into his house, he returns its value. This is an enactment to help thieves repent. (Letter of the law, he did not acquire!)

Answer (R. Zeira): A reversible Shinuy ha'Shem is not considered a Shinuy.

93b (Mishnah #1): If one stole wood and he made Kelim, he pays the value at the time he stole it.

Inference: He pays the value at the time he stole it because he made Kelim. Had he only smoothed the wood, he would not!

Contradiction (Beraisa): If one stole wood and smoothed it, he pays the value at the time he stole it.

Answer #1 (Abaye): The inference is wrong. The Mishnah teaches about a reversible change, which acquires mid'Rabanan. All the more so, an irreversible Shinuy acquires mid'Oraisa. The case is, he stole boards. The Kelim can be dissembled to boards, like he stole. The Beraisa teaches only about irreversible changes, which acquire mid'Oraisa.

Answer #2 (Rav Ashi): The inference is wrong. Also the Mishnah teaches only irreversible changes, which acquire mid'Oraisa. He merely smoothed the wood, and made clubs.

96a (Rav Papa): If one stole long beams and cut them into small beams, he did not acquire. If he made boards out of them, he acquired.

96b (Rav Papa): If one stole dirt and made a brick, he did not acquire it, because he can return it to dirt. If one stole a brick and made it into dirt, he acquired. Even though he can make a brick again, it would be a different brick.

If one stole a lump of silver and made a Zuz (coin), he did not acquire, because he can return it to a lump of silver. If one stole a Zuz and made it a lump of silver, he acquires. He can make a Zuz again, but it would not be the same as initially.

Rishonim

Rif and Rosh (34a and 9:1): We learn from Rav Papa that a reversible Shinuy is not a Shinuy; it does not acquire. This is the Halachah. We establish the Mishnah like Rav Ashi, who is Basra; he merely smoothed the wood, and made clubs.

Gra (CM 353:6): The Rif infers that since Abaye holds that a reversible Shinuy acquires mid'Rabanan, Rav Ashi must hold that it does not acquire at all. Tosfos (Sukah 30b DH Shinuy) holds that Rav Ashi explains Abaye.

Rosh: The Mishnah says 'and he made them Kelim.' This shows that merely smoothing the wood suffices only if it is called a Kli or the name changed. If one stole long beams and cut them into small beams, he did not acquire. Rabah taught that Shinuy acquires. Tosfos says that he discusses a reversible Shinuy Ma'aseh. The Gemara (93b) connotes that this does not acquire. Despair with Shinuy Reshus or with a small Shinuy ha'Shem acquires mid'Oraisa.

Rambam (Hilchos Gezeilah 2:10): A reversible Shinuy is not a Shinuy. If one stole wood and stuck them together with nails and made a box, it is not a Shinuy, for he can dismantle them and they will be boards, like they were.

Rambam (11): If one stole dirt and made a brick, he did not acquire it, for he can grind it to dirt, like it was. If one stole a strip of silver and made a coin, he did not acquire, for he can melt it back to a strip. The same applies to all similar cases.

Poskim

Shulchan Aruch (CM 353:1): A reversible Shinuy does not acquire, even mid'Rabanan.

Shulchan Aruch (2): If with despair there is a reversible Shinuy ha'Shem, some say that he acquires it and returns only its value.

Hagahos (in Tur ha'Shalem 14, citing Dibros Moshe 65:1): The Rosh said 'a weak Shinuy ha'Shem.' The Tur said 'a weak Shinuy ha'Shem that is reversible.' The Beis Yosef and Bach hold that he explains that the Rosh refers to a reversible Shinuy.

Igros Moshe (OC 3:65): Tosfos (Sukah 30b DH v'Likniyuha) holds that with despair, a reversible Shinuy ha'Shem acquires, but a reversible Shinuy Ma'aseh does not. This is because after Shinuy ha'Shem, the theft is not in the world. Even if it will return like initially, now it is not around. If so, we need not say that Abaye (93b) argues with both of his Rebbeyim (Rabah and Rav Yosef). A reversible Shinuy ha'Shem doesn't acquire by itself, but it joins with despair, which makes it as if there is no other owner. Shinuy Ma'aseh does not make it as if the theft is not in the world, so it doesn't join with despair.

Shulchan Aruch (360:5): A Shinuy acquires if it is not reversible. If one stole wood and stuck them together with nails and made a box, it is not a Shinuy, for he can dismantle them and they will be boards, like they were. If one stole dirt and made a brick, he did not acquire it, for he can grind it to dirt, like it was.

Beis Yosef (DH u'Mah she'Chosav Gezel): The Rashba (DH Ha) says that if he fired the brick in a kiln, it cannot be returned to dirt (so he acquires).

SMA (10): The Tur disagrees. Even though it cannot be returned to dirt as fine as initially, people are not so particular about this. This is unlike wool; after it was made felt, one cannot use it like normal wool.

Beis Yosef (DH v'Chosvu): Tosfos (DH v'Avdei) says that if he made a lump of silver into Kelim, he acquires, even though he could make it a lump again. This is no worse than making a Kli out of boards. A coin or brick is not so important. The Rashba disagrees. They are very important. Coins can buy anything! Making a Kli out of boards does not acquire! The Halachah follows Rav Ashi against R. Yochanan and Abaye; making a Kli from boards acquires only if he smoothed them. In any case, the thief acquires the increased value.

Shach (1): Tosfos equates this to making a Kli out of boards. It acquires according to the opinion that a reversible Shinuy acquires. Tosfos agrees that this is not the Halachah!

Shulchan Aruch (ibid.): If one stole a strip of silver and made a coin, he did not acquire, for he can melt it back to a strip. The same applies to all similar cases.

Beis Yosef (DH u'Mah she'Chosav Rabeinu umi'Ta'am): No one is particular that the strip of silver be square like it used to be.

Beis Yosef (DH u'Mah she'Chosav Rabeinu Gabei): The Tur says that if he refined old coins in a cauldron and made them new, he acquired. This is difficult, for he can make them old again! He must mean that he melted them and made new coins. This is unlike melting a strip and making coins. That does not acquire, for it is not Panim Chodoshos (a new entity). Alternatively, even if he did not melt the coins, it is recognizable that they were refined.

Beis Yosef (DH u'Mah she'Chosav Tavuy): The Rambam and Tur say that one who made spun thread into garments acquires. This is difficult, for the Gemara says that he does not!