Gemara

3b (Rav Chisda): We may not destroy a synagogue before building another one.

Opinion #1: The concern is negligence (lest they not build another).

Opinion #2: We need a place to pray until a replacement is built.

They argue in a case in which there is already another place to pray.

(Rav Ashi): Even if they already have money to build another, we are concerned lest the money be used for Pidyon Shevuyim (redeeming captives). Even if bricks and ceiling beams are ready to be assembled, we are concerned lest they be sold for Pidyon Shevuyim.

Question (Ravina): If so, even if the new synagogue is built, we should be concerned lest it be sold for Pidyon Shevuyim!

Answer (Rav Ashi): People do not sell residences.

8a (Abaye): Rav Shmuel bar Yehudah taught that we do not impose Tzedakah on orphans, even for Pidyon Shevuyim. This implies that Pidyon Shevuyim is a big Mitzvah.

(Rabah bar Mari): We learn from "whoever deserves to die (on his bed) will die (that way. Whoever deserves to die) by the sword (will die) by the sword, by hunger through hunger, la'Shevi (for captivity) la'Shevi";

(R. Yochanan): The latter punishments in the verse are harsher than the earlier ones.

Captivity is the worst, for it includes all of them.

Erchin 6a (Rav Nachman): If one said "this coin is for Tzedakah", it is permitted to change it.

(Beraisa): If one said "this coin is for Tzedakah," it is permitted to change it before the Gabai (overseer of Tzedakah) received it, but not afterwards.

Question: R. Yanai was a Gabai. He borrowed Tzedakah and repaid!

Answer: That benefited the poor. He did so to induce people to give more Tzedakah. (He could tell them that there is no Tzedakah for the poor.)

Rishonim

Rambam (Hilchos Matanos Aniyim 8:11): If people of the city collected to build a Beis ha'Keneses, and a Mitzvah arose, they may use the money for the Mitzvah. If they bought wood and rocks for the Beis ha'Keneses, they may sell them only for Pidyon Shevuyim. If they built, they finish it, and do not sell the Beis ha'Keneses. They collect for Pidyon Shevuyim from the Tzibur.

Poskim

Shulchan Aruch (YD 252:1): No Mitzvah is as great as Pidyon Shevuyim. Therefore, if money was collected for any Mitzvah, they can change it for Pidyon Shevuyim. Even if they collected to build a Beis ha'Keneses, and even if they bought the wood and rocks and set them aside to build it, in which case one may not sell them for another Mitzvah, one may sell them for Pidyon Shevuyim. If they already built the Beis ha'Keneses, we do not sell it.

Beis Yosef (DH u'Mah she'Chosav Hilkach): The Rambam learns from Bava Basra 3b. He holds that we use the money also for Tzedakos other than Pidyon Shevuyim. The Gemara mentioned Pidyon Shevuyim because it is a Mitzvah that cannot be delayed.

Gra (2): When they collected money for a Beis ha'Keneses, we are concerned lest it be used for any Mitzvah. This is like the Rambam's text. If they bought the wood and rocks, we are concerned lest they sell them only for Pidyon Shevuyim.

Shevus Yakov (2:84): A case occurred in which a man about to die commanded to give a certain amount for Pidyon Shevuyim. Now there is Dever (a plague), and many are in a dire situation. It seems that even so, one may not use the money to save from Dever. Pidyon Shevuyim is greater than this (Bava Basra 8b)! However, this was when Yisrael bestowed Chesed on each other when in need. Nowadays, there is the yoke of exile. Our enemies make false accusations that the Dever came through Yisrael, and they do not let people in or out. Some Yisre'elim hid in forests and caves. If they were found, they were open to be killed. This is the ultimate captivity! Therefore, the money may be used to save from Dever.

Beis Lechem Yehudah (1): I saw in Lita that Yisre'elim bestow Chesed with each other. However, now that most people die of starvation, and they stab each other to get the other's food and drink, this is just like captivity.

Rema: In any case, if one vowed to Tzedakah, Pidyon Shevuyim is not included. One may use the money to redeem only if the people of the city consent.

Question (Bach DH Kosav): The Maharik (7) says so, for when people say 'Tzedakah', they do not consider Pidyon Shevuyim to be included. He proves this from Erchin 6a. If one said "this coin is for Tzedakah", he may change it for his own needs, and repay later. We are not concerned lest he need it for Pidyon Shevuyim, even though we are concerned for this regarding money given to build a Beis ha'Keneses! The Tur said that money collected for any Mitzvah may be used for Pidyon Shevuyim. The Maharik must explain that this is when the congregation collected amongst themselves, with intent for what they will decide, but what individuals give may not be changed from what it was given for. Pidyon Shevuyim is not included in Tzedakah. I disagree. We are concerned lest money be needed for Pidyon Shevuyim only regarding destroying a Beis ha'Keneses, even though it is not so common, for if it will occur, there will be no place to pray. Here, we need not be concerned for Pidyon Shevuyim, which is rare. Even if it will occur, they will collect, pay immediately, redeem them, and no calamity will result. If we were concerned for this, we should require preparing money for Pidyon Shevuyim. We do not find that Chachamim did so. Further, there would be no limit how much to collect! The simple reading of the Tur connotes that any money may be used for Pidyon Shevuyim, except for money for a Beis ha'Keneses.

Defense (Shach 2): The Maharik learned from Erchin that one may change Tzedakah on condition to repay it. One may change Tzedakah before it comes to the Gabai. R. Yanai would borrow, even though he was a Gabai. This is good for the Aniyim, for when there is no Tzedakah money around, he forces others to give. However, this is bad for captives, for in any case we collect for them only their redemption, and they could die due to the delay. Every moment that one delays unnecessarily is like bloodshed! The Maharik derived that Stam Tzedakah is not for Pidyon Shevuyim. Even if when Chachamim refer to Tzedakah it is included, Tzedakah is a vow. Vows are based on how people speak (when people say 'Tzedakah', they do not include Pidyon Shevuyim). Do not say that it is included, but we are not concerned for it for it is rare. Regarding money for a Beis ha'Keneses, we are concerned for it. All the more so, we must be concerned for it and not use Tzedakah money for Reshus! This answers all the Bach's questions.

Taz (2): Why does the Tur forbid selling a Beis ha'Keneses for Pidyon Shevuyim? The Gemara does not forbid. It just says that we are not concerned for this, just like people do not sell their residences. The Rambam and Beis Yosef permit selling it until it is built, for it is still not considered a residence.

Gra (3): According to the Shulchan Aruch's text of the Gemara, one may not sell it after it was built. Our text permits.

(Aruch ha'Shulchan (2): The Mitzvah of Beis ha'Keneses is not greater than Pidyon Shevuyim. Rather, makes all efforts to avoid selling his residence, even for the greatest need. A Beis ha'Keneses, the residence of Yisrael for Torah and Tefilah, is no less. We must make all efforts to collect for Pidyon Shevuyim to avoid selling a Beis ha'Keneses. If we cannot collect, surely we sell a Beis ha'Keneses to save lives!

Shulchan Aruch (OC 153:13): If they built and finished a Beis ha'Keneses, we do not sell it for Pidyon Shevuyim. We collect from the Tzibur.

Mishnah Berurah (79,80): Until it is finished, it is like wood and rocks, which may be sold. Even after it is finished, letter of the law one may sell it, even without the seven leaders of the city. It is a mere stringency not to sell it.

Kaf ha'Chayim (118): Some texts of the Beis Yosef say 'if they built (partially), they finish.' This is when it is possible to collect from the Tzibur. If not, we sell even after it is finished, for Pidyon Shevuyim, enabling Talmidim to learn, and to marry off a man or orphan girl.