Gemara

(Mishnah): A half-slave may not eat from his master's Pesach.

88a - Inference: He may eat from his own Pesach.

Contradiction (Beraisa): He may not eat from his master's Pesach, nor from his own.

Resolution: Mishnah Rishonah (the initial version) was like the Beraisa. Mishnah Acharonah (after Beis Hillel retracted) is like our Mishnah:

(Mishnah - Beis Hillel): A half-slave serves his master for one day, and works for himself for one day;

Beis Shamai: The slave cannot marry! Rather, we force the master to free the slave.

Beis Hillel retracted, and agreed with Beis Shamai.

Eruvin 37a (Beraisa #1 - R. Meir): If one bought wine from Kusim, he says 'the two Lugim that I will separate later are Terumah...

37b (Beraisa - R. Yehudah, R. Yosi and R. Shimon, to R. Meir): Aren't you concerned lest the barrel break, and retroactively he drank Tevel [for he will never be able to separate the tithes]?

R. Meir: Until it breaks, we are not concerned.

Gitin 28a (Mishnah): A Bas Yisrael married to a Kohen may eat Terumah (even) when he goes abroad. We rely on the Chazakah that he is still alive.

Contradiction (Beraisa): If a Kohen told his wife 'this is your Get; it should take effect a moment before I die', once she takes the Get, she may not eat Terumah.

Answer #1 (Abaye): Our Mishnah is R. Meir, who is not concerned [lest the barrel break, which is like concern] lest one die. The Beraisa is R. Yehudah, who is concerned.

28b - Answer #2 (Rava): We are not concerned that the husband already died. Tana'im argue about whether or not we are concerned that he will die.

Sukah 23a (Beraisa - R. Meir): If one used an animal for a Sukah wall, it is Pasul;

R. Yehudah is Machshir.

(Abaye): R. Meir disqualifies lest the animal die.

(R. Zeira): He disqualifies lest it flee.

24a: Abaye switches the opinions. (R. Yehudah is concerned for death, and R. Meir is not.)

Nedarim 3b - Question: How can Bal Te'acher apply to Nezirus?

Once he says 'I am a Nazir' [he is immediately a Nazir]!

Answer (Rava): He said 'I will be a Nazir before I die.' He is (i.e. must become) a Nazir immediately;

This is like a Get that takes effect just before he dies. Immediately she may not eat Terumah, lest he die right away;

Also here, he is a Nazir immediately, lest he die right away!

Rishonim

Rif (Gitin 14a), Rambam (Hilchos Girushin 6:28): and Rosh (Gitin 3:4): A Shali'ach may give a Get, [even] if the Meshale'ach was old or sick. We rely on the Chazakah that he is still alive.

Perush ha'Rosh (Nedarim 3b DH Lo Eftar): He is not a Nazir until he accepts Nezirus. We learn from Terumah that it is proper to accept immediately, lest he die. Even the opinion in Gitin that argues, and is not concerned for death, is concerned here, for the days of Nezirus are a long time.

Rivash (335): If one slackens to fulfill his oath [to pay a debt], Beis Din excommunicates him, even though no time was fixed. If he fulfills it in his lifetime, he was Yotzei. Even so, the obligation to fulfill it comes immediately, like one who accepted to become a Nazir before he dies.

Rivash (198): If one swore to present his daughter for Nisu'in Stam, and did not fix a time, immediately he must fulfill this. Even though he does not transgress until it is not in his ability to fulfill, right away he must strive to fulfill without slackening. Bal Te'acher applies to Nezirus, for he must start right away, lest he die. A Beis Din fixed a time and threatened to excommunicate him if he does not marry her within the time. This was proper.

Sefer Chasidim (840): Some Chasidim would not eat a new fruit from Tamuz 17 until Tish'ah b'Av (Bein ha'Metzarim). How can we thank Hash-m for enabling us to reach this time?! Some eat it on Shabbosos Bein ha'Metzarim (from Tamuz 17 until Tish'ah b'Av). Chachamim said 'you should have blessed when you saw it, like R. Yehudah!' They say that they bless only when they eat it.

Terumas ha'Deshen (2:60): Surely, one who must fast 40 fasts for Chilul Shabbos may postpone them until winter.

Hagahos Mordechai (Bava Metzi'a 456): If one vowed to fast, and Ones occurred, he may repay on a short [winter] day, even if the vow was for a long day.

Rashi (88a DH k'Mishnah Acharonah): Since we can force the master to free him, the slave is considered free, and eats from his own Korban.

Turei Even (Chagigah 4a DH Lo): Why may a half-slave eat Korban Pesach after Beis Hillel retracted? His status is contingent. If he will die before freedom, he was a half-slave, who may not eat! Tana'im argue about concern lest one die. Why did we establish the Beraisa like Mishnah Rishonah, unlike the Halachah? We should establish it like Mishnah Acharonah, like the opinion that is concerned for death! We follow R. Yehudah and R. Shimon against R. Meir! Rather, Mishnah Acharonah must hold that even if he will never be freed, the master is not called a master. Even if he is called a master for receiving a fine (if an ox will gore the slave - Gitin 42a), for Pesach he is not called a master, because the free half of the slave is obligated. The slave half, which is contingent, cannot uproot the Chiyuv of the free half.

Poskim

Shulchan Aruch (OC 545:5): [During Chol ha'Mo'ed] one may write gift documents [and other acts of Beis Din].

Beis Yosef (DH uv'Inach): Tosfos (Mo'ed Katan 18b DH v'Elu) and the Nimukei Yosef (10a DH Masnisin) say that all of these are Devar ha'Avud, lest the giver die or Beis Din die, or go overseas.

Rema: Some permit writing a Kesuvah, and some forbid.

Magen Avraham (19): The stringent opinion forbids, for this is like a loan document (Sa'if 6). We are not concerned lest the witnesses die, for he did not yet designate witnesses. If the husband dies, all know that all women have a Kesuvah.

Nishmas Adam (1:68): R. Yehudah permits using an animal for a Sukah. Abaye says that R. Yehudah argues with R. Meir about Terumah because he holds that Ein Breirah, but he is not concerned for death. Rava holds that R. Yehudah is concerned for breakage, like it says in Gitin and Eruvin. Why is the Sukah Kosher? We should be concerned lest the animal die! Rava is concerned only when lack of concern will lead to an Isur, e.g. [if we would let her eat Terumah without] concern lest he die soon. Regarding Sukah, one will see if the animal dies!

Bi'ur Halachah (537:1 DH Devar): The Magen Avraham permits a Safek Devar ha'Avud. Seemingly, this depends on whether the Isur of Meleches Chol ha'Mo'ed that is not Devar ha'Avud is mid'Oraisa or mid'Rabanan. Nishmas Adam proves from Siman 539 that it is forbidden, even according to the opinion that forbids Melachah only mid'Rabanan. The Beis Yosef forbids according to the Rosh, who forbids only mid'Rabanan. The Mishnah (18b) seems unlike this. Tosfos and the Ritva say that it permits due to Devar ha'Avud, lest the giver, witnesses or Beis Din die or go overseas. This shows that we are lenient due to Safek! The Beis Yosef (445) brings the Rashba, who explains that the Tosefta permits writing [monetary] calculations lest he not remember them after the Mo'ed. The Rashba forbids Melachah mid'Oraisa! Rather, for a likely concern that a loss will occur, we permit, even according to the opinion that Melachah is mid'Oraisa. Tosfos says 'lest the giver, witnesses or Beis Din die [or go overseas].' I.e. due to all these possibilities together, a loss is common. For an unlikely concern lest a loss occur, we forbid, even according to the opinion that Melachah is mid'Rabanan.

Shulchan Aruch (551:17): It is good to avoid saying Shehecheyanu on a new fruit or garment Bein ha'Metzarim. If a fruit will not be available after Tish'ah b'Av, he may bless and eat it Bein ha'Metzarim.

Beis Yosef (DH Matzasi): Maharil (31) permits blessing Shehecheyanu Bein ha'Metzarim. Sefer Chasidim disagrees. I say that if it can be delayed, such as a new fruit or garment, we delay it. If Pidyon ha'Ben occurs, we do not delay the Mitzvah or Berachah.

Taz (17): R. Yosef Kohen Tzedek of Krakao permits blessing every day Bein ha'Metzarim, lest he die before fulfilling the Mitzvah! Tosfos (Mo'ed Katan 18b) supports this. He permits acts of Beis Din lest the giver die or Beis Din die. If due to this concern we are lenient for physical needs, all the more so we are lenient for Hash-m's "needs"!

Magen Avraham (44): Those who permit only on Shabbos must say that this Heter is only for a fruit that will not last until Shabbos, e.g. small cherries.

Gra (DH v'Chen): This is like the Yerushalmi says, that one will be judged [for every food that he could have eaten, but did not]. This refers to a new fruit on which he could have blessed Shehecheyanu.

Mishnah Berurah (98): Many argue with this law. Therefore, one need not be stringent on Shabbos, for in any case many are lenient on Shabbos.

Shulchan Aruch (568:3): If one vowed a certain number of fasts, he may delay them until the winter when the days are shorter.

Magen Avraham (13): He should be obligated to fast immediately, lest he die! One who vowed to buy a house must buy the first house proper for him. The Beis Yosef (CM Sof Siman 75) brought from the Rivash (335, 344) that one must pay a vow immediately, lest he die. Even though Bal Te'acher applies to Nezirus only if he said 'I will be a Nazir before I die', this is because there is no other case. If one vows to fast one day, he must fast immediately, lest he die, and all the more so if it is a fast for Teshuvah, lest he die without Teshuvah. Hagahos Mordechai permits in a case of Ones to borrow from a long day to a short day. This implies that l'Chatchilah it is forbidden. However, for a short time we are not concerned lest he die, like the Rosh says in Nedarim.

Mishnah Berurah (26): The same applies to one who fasts for Teshuvah.

Mishnah Berurah (28): The Magen Avraham permits only due to Ones. If not, it is forbidden lest he die, unless it is a short time until the winter. Also Machaneh Efrayim (Hilchos Nedarim 15) says that one must fulfill immediately. However, if he says that he did not intend to fast immediately, he is believed.

Rema (YD 232:12): If one swore or vowed to do something within a year, or on a certain day, he must do it immediately, or in the morning of that day, lest he forget to do it, and transgress his oath or vow.

Darchei Moshe (7, DH Kasav ha'Ran): The Rivash (198) says that one must fulfill a vow once he can. He may not say 'I still have time, for I did not fix a time.' He may not slacken, lest he be unable to fulfill afterwards, or lest he die. This is why one who accepted to become a Nazir before he dies must become a Nazir immediately.