Gemara

(Mishnah): A widow collects her Kesuvah from orphans' property only by swearing (that she did not already receive her Kesuvah).

Judges refrained from administering the oath. R. Gamliel enacted that she vow like the orphans desire, and then collect.

35a - Suggestion: They refrained from giving the oath due to cases like Rav Kahana recorded. (Once, Levi deposited a coin with a widow. Unknown to her, it was baked into a loaf, which she gave to a poor man. When Levi asked for his coin, she swore that one of her children should die if she benefited from it. One of her children died shortly after this. (The coin saved her flour that otherwise would have been used in the loaf.)

Objection: If so, they should have stopped divorcees from swearing as well!

(R. Zeira): They stopped only widows from swearing. Divorcees still swear.

Answer: A widow toils to care for the orphans. She might rationalize swearing that that she did not receive any money from the Kesuvah, even if she did, since she feels that she is entitled to compensation for her toil.

(Rav Yehudah citing Shmuel): She does not swear in Beis Din, but she may swear outside of Beis Din.

(Rav Yehudah citing Rav): She does not swear even outside of Beis Din.

This is why Rav did not allow widows to collect the Kesuvah. Rav did not allow them to collect through a vow, for in his day, people took vows lightly.

Rav Yehudah (to R. Yirmeyah Bira'ah): Make the widow vow in Beis Din and swear outside of Beis Din. Publicize that widows collect Kesuvos!

Rishonim

Rif and Rosh (4:8): Shmuel permits her to swear outside of Beis Din. Rav forbids. Rav did not allow a widow to collect her Kesuvah, not even through a vow, for in his day, people took vows lightly. Rav Yehudah said that he will make the widow vow in Beis Din and swear outside of Beis Din to publicize the matter. This applies to widows. Divorcees may swear.

Ran (DH Masnisin): We are not concerned lest she transgress her vow. Even if she does, the punishment is less than for false oaths, for which the one who swore and the entire world are punished. The Yerushalmi says that women used to swear and bury their children. Also, people fear vows more than oaths. The Gemara mentioned a vow to forbid all food. This is not precise. An impossible vow is like an oath not to sleep for three days. He is lashed (for a vain oath) and sleeps immediately (he need not try to fulfill the oath as long as possible). The vow must permit her to eat something in order to live.

Ran (Kesuvos 21b DH li'Shvu'ah, cited in Darchei Moshe 12): If she swore about the Kesuvah, she need not swear about additions to the Kesuvah.

Rosh: Rav Yehudah said that he will make the widow vow in Beis Din or swear outside of Beis Din. He holds like his Rebbi, Shmuel, for the Halachah follows him in monetary matters. R. Chananel agrees. The Ri says that nowadays we make widows swear even in Beis Din. Our 'oaths' are decreeing on her with Cherem and an oath that she will admit to whatever she received from her Kesuvah. The punishment is smaller than for an oath that is false once she says it. That is Shevu'as Shav, about which it says "Hash-m will not cleanse." We find that if one once swore falsely (immediately), he is suspected to swear falsely, but not for a Shevu'as Bituy (about the future) that he did not fulfill and retroactively was false. Perhaps he swore truthfully (i.e. with intent to fulfill it)! R. Tam says that even one who did not fulfill a Shevu'as Bituy is suspected to swear falsely, but he could agree that the punishment is greater for an oath that is immediately false.

Rambam (Hilchos Ishus 16:11): When a widow comes to collect her Kesuvah, Beis Din or the orphans make her swear only outside of Beis Din. Batei Din refrained from making her swear, lest she not be careful about the oath. If the orphans want, they may force her to vow to them anything they want. We do so in Beis Din, and then she receives her Kesuvah.

Magid Mishneh: Rashi, the Ramban and Rashba hold that outside of Beis Din is like a Shevu'as Heses, without Hash-m's name or a (Kodesh) object (e.g. Sefer Torah). It seems that the Rambam disagrees, for he did not specify.

Darchei Moshe (EH 96:13): Hagahos Maimoniyos and the Mordechai also connote that she must hold an object.

Poskim

Shulchan Aruch (EH 96:19): When a widow comes to collect her Kesuvah, Beis Din or the orphans make her swear only outside of Beis Din. Batei Din refrained from making her swear, lest she not be careful about the oath. An oath out of Beis Din is not so severe. It is without Hash-m's name and without holding an object. It is a mere curse. The vow must be a matter or affliction for her entire life.

Beis Yosef (DH uvi'Gemara): Rashi says that inside Beis Din is a mid'Oraisa oath with Hash-m's name or a Kinuy (another name He is called) and holding a Sefer Torah or Tefilin. Outside of Beis Din is a curse without holding anything. The punishment is not so great. The Rif, Rambam and Rosh rule like Rav Yehudah. The orphans decide if they want her to swear outside of Beis Din, or to vow in Beis Din.

Shulchan Aruch (21): Some say that nowadays we make widows swear even in Beis Din. Our 'oaths' are decreeing on her with Cherem and an oath mid'Oraisa that she admit to whatever she received from her Kesuvah. The punishment is not as great as when she herself says a false oath. That is Shevu'as Shav, about which it says "Hash-m will not cleanse."

Beis Yosef (DH u'Mah she'Chosav b'Shem): Tosfos, the Rosh, the Mordechai and R. Yerucham all bring this, from the Ri.

Taz (21, citing the Mordechai): They used to make her swear that what she says now is true. If she lies, her Shevu'ah is Shav. Nowadays she swears that she will answer our questions truthfully. If she lies, retroactively her Shevu'ah was Sheker. When she swore, she intended that it will be true. The Shulchan Aruch says that we impose an oath that she will admit. Perhaps she plans to admit years later! Rather, Beis Din should tell her that we will ask her immediately. Some make her hold a Sefer at the time of the oath. This is wrong; the Beis Yosef did not bring this.

Rema: She must swear in front of three Kosher judges. If she swore in front of three relatives, she must swear again.

Beis Shmuel (39): Normally, one does not bequeath an oath (if he needed to swear to collect, his heir cannot swear and collect). If she needed to swear again in front of Kosher judges, and died, her heirs can swear.

Gra (37): Some obligate an oath outside of Beis Din and a vow in Beis Din.

Rema: L'Chatchilah, she must swear in front of the orphans. If she swore in their absence, or they do not want to be there, she swears in their absence (and collects). Even after she swore, the orphans or Beis Din can impose a Cherem on anyone who knows that she deposited their property with others.

Source (Rivash 325, cited in Darchei Moshe 12): If such witnesses will come, we take the property from her. She need not accept the Cherem.

Beis Shmuel (40): The orphans can impose this Cherem only if they know that witnesses know about this.