Gemara

If goats [bought for Chata'os of the Tzibur] were leftover, R. Shmuel bar Rav Yitzchak says that we offer them for Kitz ha'Mizbe'ach (Olos Tzibur, when the Mizbe'ach is idle).

Question: Can we offer a Chatas for an Olah?!

Answer #1 (R. Yosa): Korbanos Tzibur are fixed (which Korban they will be) only at the time of Shechitah.

Answer #2 (R. Chiya): It is a Tenai Beis Din that the leftovers be offered for Olos.

Shevuos 11a (Rabah): Beis Din purchases things needed for the Avodah on condition that anything that is not needed does not get Kedushas ha'Guf.

Question (Abaye - Mishnah - R. Yehudah): If the bull or goat of Yom Kipur was lost, and another was taken in its place and the lost animal was found, we leave it to die. (If Hekdesh buys on condition, we should redeem it!)

Answer (Rabah): These are uncommon Korbanos (and it is rare that one would be lost). Hekdesh does not stipulate about them.

Megilah 26b (Beraisa): Genizah is required for Tashmishei Kedushah, e.g. sacks for Seforim, Tefilin, and Mezuzos, cases of a Sefer Torah or Tefilin, and Tefilin straps.

Kidushin 54a (Rav): Me'ilah does not apply to Bigdei Kehunah, for the Torah allows Kohanim to benefit from them. Kohanim are not like angels who could instantaneously remove them after serving in the Mikdash!

Yoma 59a: Tana'im argue only argue about whether or not Me'ilah applies to blood mid'Rabanan. All agree that mid'Oraisa Me'ilah does not apply.

Rishonim

Rosh (Megilah 4:11): In the Yerushalmi, R. Yirmeyah asked about putting a Keli in the Aron, for people used to do so. R. Mana said that they stipulated to do so from the beginning.

Terumas ha'Deshen (273): If there was no initial Tanai (stipulation), surely one may not benefit from the Aron, cloths and covers of the Sefer Torah. We are not careful, even without a Tanai. We must say that this is because it is almost impossible to be careful. Me'ilah does not apply to Bigdei Kehunah, for Kohanim are not like angels. Even though Rashi connotes that we permit Hana'ah only b'Shogeg, we hear that such matters are considered impossible to be careful about. If so, Lev Beis Din stipulates about it, lest people transgress. One who donates such things to the Beis ha'Keneses authorizes the leaders of the city or Beis ha'Keneses over them, so there is such a stipulation. We say so about extra lambs for the Tamid. Rashi says that Beis Din stipulates about all matters of the Tzibur, i.e. what is common. I was pressed to justify why people are not careful about these matters.

Tosfos (Yoma 59b DH Ad): The Gemara infers that Me'ilah of blood is mid'Rabanan, for if it were mid'Oraisa (the blood still has Kedushas ha'Guf), it would be forbidden to sell it to gardeners!

Rebuttal (Ri Korkus Hilchos Me'ilah 2:11 (The blood lost Kedushas ha'Guf. After it goes to Nachal Kidron, Beis Din gave it Kedushas Damim.) It is redeemed and sold to gardeners, like other Hekdesh Bedek ha'Bayis. Alternatively, Beis Din stipulates that after it is sold, only the money will be Kodesh.

Terumas ha'Deshen (316): Reuven used to rent out his horse. Shimon needed to borrow it to save a big loss. Reuven's wife said 'Reuven needs it today to avoid a (smaller) loss.' Shimon said that he will pay the rent and compensate Reuven if Reuven loses. Shimon is a Sho'el she'Lo mi'Da'as, who is a thief. The Rosh derives this from Bava Kama 60b. However, many hold that he is not a thief, for Beis Din stipulated that Yosef may force Moshe to help save Yosef from a loss (Bava Kama 81b). If so, all the more so Yosef may take what he needs to avoid a loss! The Rif rules that there is no such enactment.

Question: Since Me'ilah applies to worn out Bigdei Kehunah, how could they be used for Simchas Beis ha'Sho'evah? It is not a need of Korbanos!

Suggestion: Perhaps Beis Din stipulates to permit this.

Rejection (Tosfos Shabbos 21a DH Simchas): There is no reason for Beis Din to stipulate thusly.

Answer (Tosfos ibid.): Since it is to honor Nisuch ha'Mayim, this is considered a need of the Korban - "u'Sh'avtem Mayim b'Sason".

Mishneh l'Melech (Hilchos Klei ha'Mikdash 8:6): I.e. honoring Nisuch ha'Mayim is considered a need of the Korban to justify a stipulation of Beis Din.

Poskim

Shulchan Aruch (OC 154:8): A Tanai helps for the Aron and everything we make for a Sefer Torah, to use it for other things, even Chulin.

Taz (7): It seems that Kedushas Aron is only when it is made to always house a Sefer Torah. If it is made temporarily, e.g. nowadays that many Batei Kenesiyos became desolate, and when Yisrael return they make a small Aron for the Sefer Torah until they can make a bigger Aron. Once the latter Aron is made, the former loses its Kedushah. This is like a Beis ha'Keneses that was rented temporarily. It does not get Kedushas Beis ha'Keneses, therefore, one may put Seforim inside. It seems that Lev Beis Din stipulates also about this.

Rema: The custom is to get many benefits from Divrei Kedushah, e.g. cloths of Seforim and the table in the Beis ha'Keneses and covers of the Sefer Torah. They say that since this is the custom, and it is impossible to be careful about this, Lev Beis Din stipulates from the beginning, in order that people not stumble. Even if they did not stipulate, it is as if they stipulated.

Beis Yosef (DH v'Zeh): Why may we put Sidurim and other Seforim on the cloth on the Bimah (on which the Sefer Torah is read)? We must say that since this is the custom, they stipulated from the beginning. Even if they did not stipulate, we can say that Lev Beis Din stipulated.

Mishnah Berurah (35): It is clear from the Terumas ha'Deshen that the Heter to benefit, because one cannot be careful, applies only in a Beis ha'Keneses, but not to an individual's Sefer Torah.

Bi'ur Halachah (DH v'Iy): The Rema connotes that we do not assume that Beis Din stipulated about things that one can be careful about.

Kaf ha'Chayim (69): If Beis Din stipulated about something we could be careful about, the Tanai helps.

Shulchan Aruch (OC 42:3): If a cloth was permanently designated to wrap Tefilin, and one wrapped Tefilin in it once, one may not wrap coins in it.

Rema: If he stipulated from the beginning, it is permitted in every case.

Taz (1): This implies that an explicit stipulation is needed. Many people, when going on a trip, put things that hey will need in the Tefilin bag. This is improper. Perhaps Stam is as if they explicitly stipulated. The Rema (154:8) says that the custom is to get several benefits from Divrei Kedushah. Since it is the custom and people cannot be careful about this, Beis Din stipulates about this from the beginning. Likewise, one who buys a Tefilin bag intends to use it like people do when traveling.

Shulchan Aruch (569:1): If an individual was fasting due to an affliction, and it passed, or for a Choleh, and he healed or died, he must complete all the fasts he accepted. However, if a Tzibur was fasting, and they were answered before midday, they do not complete the fast.

Mishnah Berurah (1): A Tzibur is different, for Lev Beis Din stipulates for them (Magid Mishneh Hilchos Ta'aniyos 1:16). Also, we do not burden the Tzibur. An individual must fulfill his acceptance, since he did not explicitly stipulate not to [fulfill it] if the affliction will pass.

Taz (1): Even if he intended for this, it is Devarim sheb'Lev.

Rema (CM 308:7): If Ploni took David's donkey without permission and used it with intent to pay rental, he is Sho'el she'Lo mi'Da'as, who is like a thief. If the donkey is normally rented, he is not a thief, unless David's household protested that David needs it. If Levi did so to avoid a loss, and will compensate David for his loss, he is not a thief, for Beis Din stipulated about this.