Laws of Robbery or Loss, chapter 14, law 6. Rav Shach asks a question on Tosphot in Bava Kama page 69a. According to Tosphot, a person that steals from a robber (after the owner has already given up on getting the object back), is liable to repay four or five. (A thief that steals a sheep is required to repay four sheep and if he stole an ox, he is required to repay 5 oxen). Rav Shach asks on this how is this possible. it has already gone out of the possession of the owner what Rav Shach intends to ask here is in a case where a robber has taken an animal and the owner has given up on getting it back. then a thief steals it from the robber. At that point there has been giving up and change of possession, and therefore the thief owns the animal. So, if he sells or slaughters the animal, he ought not be liable for 4 or 5. According to the Gemara on page 68 a, there the gemara holds that even though a thief is usually liable for four or five, however if he already owns the animal, he is not liable for that amount. After all, he is selling what already belongs to him or slaughtering what already belongs to him. The answer to this question I believe is found on page 68b in the second approach of the Gemara. There is an argument between R. Yochanan and Reish Lakish if a thief sells an animal after the owner has given up. Reish Lakish said he is not liable because he is selling what already belongs to him. R Yochanan disagrees with this logic and states that he is liable. So, we see that R. Yochanan does not hold with the reasoning of “he is selling what already belongs to him,” and therefore is not liable. So, we can answer the question of Rav Shach on Tosphot. Here, Tosphot is going like R. Yochanan and therefore even a thief of a robber after the owner has given up the thief is still liable to repay four or five.------------------------הלכות גזלה או אבדה פרק י''ד הלכה ו'. רב שך asks a question on תוספות in בבא קמא דף ס''ט ע''א. According to תוספות, a person that steals from a robber (after the owner has already given up on getting the object back), is liable to repay four or five. (A thief that steals a sheep is required to repay four sheep and if he stole an ox, he is required to repay 5 oxen). רב שך asks on this how is this possible. it has already gone out of the רשות of the owner. What רב שך intends to ask here is in a case where a גזלן has גזל an animal, and the owner has given up on getting it back. Then a גנב steals it from the גזלן. At that point there has been giving up and change of possession, and therefore the גנב owns the animal. So, if he sells or slaughters the animal, he ought not be liable for four or five. According to the גמרא on page ס''ח ע''א, that even though a גנב is usually liable for four or five, however if he already owns the animal, he is not liable for that amount. After all, he is selling what already belongs to him, or slaughtering what already belongs to him. The answer to this question I believe is found on page ס''ח ע''ב in the second approach of the גמרא. There is an argument between ר' יוחנן and ריש לקיש if a thief sells an animal after the owner has given up. ריש לקיש said he is not liable because he is selling what already belongs to him. ר' יוחנן disagrees with this logic, and states that he is liable. So, we see that ר' יוחנן does not hold with the reasoning of “he is selling what already belongs to him,” and therefore is not liable. So, we can answer the question of רב שך on תוספות. Here, תוספות is going like ר' יוחנן and therefore even a גנב of a גזלן after the owner has given up the thief is still liable to repay four or five.