(a) Several Rishonim ask your question - why is the Sukah built with stolen sechach or a stolen wooden beam not invalid because it is a Mitzvah HaBa'ah b'Aveirah?

(1) HaMaor Hagadol (14b in Rif pages) answers that the Halacha does not follow Rabbi Yochanan in the name of Rabbi Shimon bar Yochai (above 30a) that a M.H.b'A is invalid. Rather the Halacha conforms with Rabbi Yitzchak bar Nachmani in the name of Shmuel (ibid.) that on the second day Yomtov, since a person fulfils his Mitzvah with a borrowed Lulav, he also fulfils the Mitzvah with a stolen Lulav.

(2) Hasogas HaRa'avad (printed on the same page as the aforementioned HaMaor) answers that when someone throws his friend out of his Sukah, this is not considered a M.H.b'A because of the principle that "Karka Eina Nigzeles" - it is not possible to steal land, i.e. land does not change ownership when it is stolen. This means that the Sukah is considered as a Sukah built on borrowed land, not on stolen land. Ra'avad writes that similarly when one steals wood and makes a Sukah with it, this wood is not considered as a stolen object. It seems to me that the Ra'avad means to say that this is not considered as a stolen article because when the thief pays the value of the wood back to the original owner, this means that he now acquires the wood, so he is not now sitting in a Sukah built from stolen articles. The transgression of stealing the Sukah has already been committed so once he actually performs the action of the Mitzvah it is not the transgression which is now enabling him to perform the Mitzvah. This is not similar to someone who steals a Lulav, and did not make any change in the Lulav, which is therefore considered M.H.b'A, because in the latter case at the time he does the Mitzvah the transgression is still noticeable in the Lulav, because it is even at this moment a stolen Lulav.

(3) The Ritva answers in the name of Tosfos (this answer does not appear in our Tosfos) that the principle of M.H.b'A only applies for Mitzvos through which we appease Hash-m and try to make ourselves acceptable to Him through these Mitzvos. For instance, when someone offers a sacrifice or blows the Shofar on Rosh Hashanah, or shakes the Lulav, these are all Mitzvos which we do in order to make ourselves acceptable to Hash-m. In contrast, the latter concept does not apply for the Mitzvah of Sukah. Note 227 to Ritva cites Tosfos Rashba Pesachim 35b DH Aval who writes that when we shake Lulav or bring a Korban, we thereby are praising and thanking Hash-m. This is similar to Ritva's answer in the name of Tosfos. However, sitting in a Sukah is not considered an act of praising Hash-m.

(4) Ritva himself disagrees with Tosfos and gives an answer similar to that of Ra'avad - above(2). The Sukah never belongs to the thief. This is similar to someone who built a Sukah on stolen land. Even though he did an Aveirah by stealing the land, nevertheless the Mitzvah of Sukah is seperate from the transgression of stealing the land. He is not performing a Mitzvah with a stolen article, so this is not considered a M.H.b'A.

(b) In fact Tosfos DH Aval writes that it is because of the principle of Takanat HaShavim that the thief does not have to demolish the Sukah in order to return the stolen sechach or beam, but it is sufficient to return the value of the stolen article to the original owner.

KOL TUV

Dovid Bloom