Gemara

(Beraisa): If Reuven went overseas, and his father heard that Reuven died, and got up and wrote all his property to others, and later, Reuven returned, the gift stands;

R. Shimon ben Menasiya says, the gift is void. Had he known that his son was alive, he would not have given it.

(Rav Nachman): The Halachah follows R. Shimon ben Menasiya.

146b (Mishnah): If a Shechiv Mera (one who fears lest he die from his illness) gave away all his property and retained any amount of land for himself, he cannot retract (it is like a gift of a healthy person). If he did not keep land for himself, he can retract.

(Rav Nachman): Our Tana is R. Shimon ben Menasiya, who follows Umdena (estimation of intent).

149b Question: What it considered 'any amount'?

Answer #1 (Rav Yehudah): It is land to support himself from it.

Answer #2 (R. Yirmeyah bar Aba): It is enough Metaltelim to support himself.

(R. Zeira): These are great teachings! If he left over any amount of land, he cannot retract, for he made provisions to support himself (in case he will live). The same reasoning applies when he leaves over Metaltelim!

Objections (Rav Yosef): The Mishnah discusses leaving over only land, but not Metaltelim! The Mishnah discusses leaving over any amount, even if it is not enough to support himself!

Answer (Abaye): Sometimes a Mishnah says 'land' (or 'any amount'), but it also applies to Metaltelim (and there is a Shi'ur! Abaye gave exampes of this.)

Rebuttal (Rav Yosef): Those Mishnayos say 'land' and 'any amount' for parallel structure or contrast to the Reisha.

150b (Rava citing Rav Nachman): Five laws apply only if one gave away all his property: a Shechiv Mera, a slave, a wife, sons, and a gift to evade.

Rishonim

Rif (70a): Rav Yehudah said that a Shechiv Mera must leave enough land to support himself from it. The Halachah does not follow him, for Rava said in the name of Rav Nachman that Matanas (the gift of a) Shechiv Mera applies only if he gave all his property. This shows that any amount is a Shiyur (remnant).

Rebuttal (Rosh 9:26): The only one who disagrees with Rav Yehudah, R. Yirmeyah bar Aba, and R. Zeira, who praised their teachings, is Rav Yosef. Abaye challenged Rav Yosef, and Rav Yosef's answer was inadequate.

Note: The Gra (CM 250:20) nearly copies the Rosh's words, but says that Rav Yosef merely questioned why R. Zeira praised their teachings, but Rav Yosef himself did not argue with their teachings. This is unlike our text of the Rosh, which says 'Abaye was astounded that Rav Yosef was astounded by their words.'

Rosh (ibid): The Rif assumes that the same Shiyur applies to all five matters. This is wrong. Metaltelim is a Shiyur for everything except for Kesuvah, which was enacted from land. Likewise, there is good reason why the Shiyur for Shechiv Mera is enough to live on! If he left less, surey he intended to give only if he will die! For a farmer, the Shiyur is enough to support his family. If he does business or lends (to Nochrim) on Ribis, the Shiyur is enough to support himself from the profits.

Rambam (Hilchos Zechiyah 6:1): We always estimate the intent of the giver. If circumstances indicate his intent, we follow Umdena, even if he did not specify. If Reuven went overseas, and his father heard that Reuven died, and give all his property for an open, absolute gift to someone else, and later, Reuven returned, the gift is void. Clearly, had he known that his son was alive, he would not have given all his property. Therefore, if he left over anything from his property, be it land or Metaltelim, the gift stands.

Magid Mishneh: Rav Nachman taught five laws that apply only if one gave away all his property. He did not need to teach about this case, for he taught Shechiv Mera, and Rav Nachman (146b) equated this case to Shechiv Mera.

Gra (CM 246:2): In one answer, Tosfos (150b DH v'Ilu) says that Rav Nachman did not list matters about which Tana'im argue.

Rambam (8:15): If a Shechiv Mera left anything for himself, be it and or Metaltelim, this is a partial gift, and the giver cannot retract.

Poskim

Shulchan Aruch (CM 246:1): We always estimate the intent of the giver. If circumstances indicate his intent, we follow Umdena, even if he did not specify. If Reuven went overseas, and his father heard that Reuven died, and give all his property for an open, absolute gift to someone else, and later, Reuven returned, the gift is void.

SMA (1): This excludes a concealed gift.

Shulchan Aruch (ibid): Clearly, had he known that his son was alive, he would not have given all his property. Therefore, if he left over anything from his property, be it land or Metaltelim, the gift stands.

SMA (2): All agree that any amount is a Shiyur. This is unlike the gift of a Shechiv Mera. There (CM 250:4), one opinion says that he can retract only if he kept enough to live on.

Rebuttal (Taz): The Rosh, who holds that a Shiyur for a Shechiv Mera must be enough to live on, holds similarly here. The Gemara (146b) said that R. Shimon ben Menasiya, who follows Umdena here (that he gave his property to a stranger only because he thought that his son was dead), holds that Matanas Shechiv Mera with a Shiyur is like a healthy person's gift. There is an opinion that there is a greater Umdena here than regarding Shechiv Mera (that he gave his property only because he expected to die). The Umdena of Shechiv Mera applies unless he left enough to live on, so all the more so the Umdena here applies unless he left this much. The Rosh says that unless a Shechiv Mera left this much, surely he did not expect to starve and die. All the more so here, surely he did not abandon his sons, unless he left this amount for his son. Also the Magid Mishneh equates them.

R. Akiva Eiger: The Gemara says 'he heard... and got up and wrote...' Perhaps this shows that there is an Umdena only if he did so right after he heard (see Tosfos Kesuvos 109a DH Teshev). Lechem Rav (205) says that there is an Umdena only when he gave to a stranger, but not if he gave to one of his sons.

Shulchan Aruch (250:4): If a Shechiv Mera left anything for himself, be it land or Metaltelim, this is like a gift of a healthy person.

Beis Yosef (DH u'Mah she'Chosav ba'Meh, citing the Rashbam): If he did not leave any land, surely he gave only because he expected to die. He did not intend that if he recovers, he will starve to death and need to take from others!

Beis Yosef (DH u'Mah she'Omar): The Rambam holds like the Rif. They must explain that if he left less property than needed to live from, he intends to live very meagerly.

SMA (16): The Shulchan Aruch equates this to other matters, e.g. one who wrote his property to others. Sometimes a person is able to support himself from any amount. If brothers divide and we find that one has great wealth, we say that perhaps he economized on food and saved up!

Rema: Some say that he must leave enough to live on.

SMA (17): The Tur says in the name of the Rosh that for a farmer, the Shiyur is enough to support his family. If he does business or lends on Ribis, the Shiyur is enough to support himself from the profits. Ba'al ha'Itur suggests that 'enough to live on' is income for 12 months, like we say about a big estate (139b). Ir Shushan says that the Rosh agrees that this is the Shi'ur. I disagree. The Rosh said 'enough to support himself from the profits.' If the profit from the principal can finance him for 12 months, surely it can finance him (forever) after this! It is difficult to say that he commanded that the principal be taken after 12 months. Ba'al ha'Itur learns 12 months from Rav, but the Halachah follows Shmuel, that a big estate is enough to feed the orphans until Bagrus or Kidushin of the girls. The Rema says 'enough to live on', which connotes permanently, like the Rosh. If there is any doubt, ha'Motzi mi'Chavero Alav ha'Re'ayah; the giver is Muchzak in the property, and the recipient must prove that the gift was valid. The Rema did not say 'enough to support his family', like the Rosh said. Do not say that he did not need to specify. Regarding Sidur (if one owes, we leave him with what he needs) for one who owes Erchin or a debt, we do not leave for needs of his family (CM 97:23)! It seems that the Rema holds that the Rosh requires leaving enough to support his family only when he left land, for one cannot plow, plant, harvest, thresh, and do other difficult farming chores all by himself. The Rema discusses business and related profits, which do not require others' help.