Gemara

50a (R. Ila'a): In Usha it was enacted that one should not give more than a fifth of his income (to Tzedakah), lest he come to need to take from people.

Support (Beraisa): One should not give more than a fifth of his income to Tzedakah, lest he come to need to take from people;

Version #1: A case occurred in which a man wanted to give more. His colleague, R. Yeshevav, did not allow him.

Version #2: R. Yeshevav wanted to give more. His colleague, R. Akiva, did not allow him.

(Rav Nachman): We learn from "Aser A'asrenu (I will tithe twice)".

Question: The second tithe (is a tenth of the 90% remaining after the first tithe) is less than a tenth. In all, less than a fifth is given!

Answer (Rav Ashi): "A'asrenu" - I will take the second tithe (on it, i.e. all that You give to me,) like the first tithe.

67b: When Mar Ukva was dying, he asked to see his Tzedakah records. He saw that he had given 7000 good Dinarim. (One can live for a year with 200 Zuz - Tur YD 253.)

Mar Ukva: I have sparse provisions for a long journey!

He gave half his money to Tzedakah.

Question: R. Ila'a taught that in Usha they enacted that one should not give more than a fifth of his money to Tzedakah!

Answer: That applies only during his life, lest he come to need. This is not a concern when he dies!

Rishonim

Rif and Rosh (18a and 4:15): In Usha they enacted that one should not give more than a fifth of his money to Tzedakah. That applies only during his life, lest he come to need. This is not a concern when he dies, like the episode with Mar Ukva.

Rosh and Ran (DH Hu): The Yerushalmi says that the first time he gives a fifth (of his principal). Afterwards, he gives a fifth of his income every year.

Rambam (Hilchos Matanos Aniyim 7:5): If an Oni requests all he needs, and the giver cannot afford this, he gives according to his ability. Ideally, he should give up to a fifth of his property. It is average to give a tenth. It is stingy to give less than a tenth.

Ri Korkus: We learn from Kesuvos 67b that one should not give more than half at the time of death. Why did the Rambam omit this? The Rambam did not specify what happens after giving a fifth. It seems that one is obligated to give to an Oni what he needs, up to a fifth. If another Oni comes later, one gives his needs up to a fifth of what remains. If Oniyim do not come, one uses a fifth of his principal for Mitzvos, and a fifth of all future profits.

Bach (DH Shi'ur): The Rosh brings the Heter to give more than a fifth at the time of death. Why did the Tur omit it? It seems that he follows the Rambam, who also omitted it. Perhaps they hold that we cannot learn a Shi'ur from Mar Ukva. Perhaps he gave only half because he used to give a fifth every year, but one who gave less very year may give more than half at the time of death. Alternatively, perhaps Mar Ukva gave half because he knew that he leaves Berachah to his heirs, but others should leave more than half to their heirs. The Rambam and Tur omitted the law, so everyone will do according to his wealth and the situation of his heirs.

Kesef Mishneh (Matanos Aniyim 7:5): The Rambam discusses only giving during one's lifetime.

Rambam (Hilchos Erchin 8:13): One should not make all of his property Hekdesh or Cherem. One who does so transgresses the Torah's intent. "(One may be Makdish) mi'Kol Asher Lo", not Kol Asher Lo. This is not Chasidus, rather, lunacy. He wastes his money and will need to take from others, and they will not have mercy on him. Rather, one should spend at most a fifth of his money on Mitzvos.

Perush ha'Mishnayos (Pe'ah 1:1 DH u'Gemilus): There is a Shi'ur to monetary Chesed. It is Midas Chasidus to give a fifth. If David encounters a captive who needs to be redeemed, or someone without food or clothing, if more than a fifth of David's money is needed, he gives a fifth. He does not transgress for not giving more. If he does not find such people, he separates a fifth of his earnings, not his principal, and gives it for the needs of Mitzvos.

Shitah Mekubetzes (Kesuvos 50a DH ha'Mevazvez): The Medrash says that R. Tarfon gave 600 Kintarei to R. Akiva (it seems that this is more than Avraham paid for Ma'aras ha'Machpelah - Bava Metzi'a 87a). R. Akiva questioned why he gave more than a fifth. R. Tarfon answered "Pizer Nosan la'Evyonim v'Tzidkaso Omedes Lo'ad" - if one transgressed and gave more than a fifth, he will always have enough to give Tzedakah. Perhaps this is only for the sake of Talmud Torah. Perhaps this is why Rashi says that one should not give more than a fifth to Aniyim. R. Yonah says that one may do as he wishes at the time of death. We find that Mar Ukva gave all of his money. (Note: this is unlike our text.) It seems that even at the time of death one should not give more than a fifth

Shitah Mekubetzes (DH Mai): One should give a fifth of his income for Ma'aser. The Torah wrote "Aser Ta'aser" to teach that it is better to give part at a time than all at once. Also, one Ma'aser may be used for any Mitzvah, even for Seforim or other Mitzvah needs of his household. We learn from Ma'aser Sheni, which one keeps for his family. The other Ma'aser is given to the poor, e.g. Kohanim and Leviyim who engage in Torah. One who is meticulous about Ma'aser is guaranteed to succeed financially.

Poskim

Shulchan Aruch (YD 249:1): If one can afford, he should Tzedakah according to the needs of the poor. If he cannot afford this, ideally he should give up to a fifth of his property. It is average to give a tenth. It is stingy to give less than a tenth.

Gra (2): The Yerushalmi says that Ma'aser Oni applies also to money.

Shulchan Aruch (ibid.): Giving a fifth means refers to a fifth of the principal the first year, and afterwards a fifth of his income every year.

Beis Yosef (DH v'Kamah): The Gemara did not say that it is a Mitzvah to give a fifth, just that one should not give more. The Yerushalmi (Pe'ah 1:1) connotes that it is a Mitzvah. The Bavli connotes similarly. We learn a fifth from "Aser A'asrenu".

Birkei Yosef (DH Oh): Rishon l'Tziyon says that one gives a Ma'aser after deducting taxes, just like one deducts taxes before giving (one part in 60 for) Pe'ah. It is not clear that Ma'aser is the same, for we learn Pe'ah from a verse. Rishon l'Tziyon leans to allow deducting taxes from Ma'aser.

Rema (249:1): One should not give more than a fifth lest he need to take from people. This applies during his life. When he is dying, he can give as much as he wants.

Mor u'Ktzi'a (brought in Hagahos Tur ha'Shalem 9): We learn from Mar Ukva that one may give much Tzedakah before dying. This is not considered averting inheritance. Presumably, Mar Ukva had children. Even if he didn't, every Yisrael has heirs!

Rema: One should not use his Ma'aser for a Mitzvah, such as lamps for the Beis ha'Keneses. It should be given to the poor.

Taz (1): If a person bought a Mitzvah in the Beis ha'Keneses and intended at the time to use Ma'aser money, it is permitted, for the money goes to Tzedakah. Even though he benefits, i.e. he can honor people to read from the Torah, there is always Tovas Hana'ah in Ma'aser. (One chooses to which Aniyim to give). However, if one bought a Mitzvah without intent to use Ma'aser money, he may not decide later to use Ma'aser. This is like paying a debt from Ma'aser.

R. Akiva Eiger: The SHLaH permits to give from Ma'aser only the amount that one added above the previous bid, for only this is added to Tzedakah (in any case the previous bid would have been given).

Taz (1): The Drishah (1 DH Asher) permits using Ma'aser for a Mitzvah such as Bris Milah, bringing a Chasan and Kalah to Chupah, or buying Seforim to learn and lend to others, if he could not do so without the Ma'aser. It seems that he must write in the Sefer that it was bought from Ma'aser, lest his children think that the Sefer is theirs.

Shach (3): Maharam me'Rotenburg permits giving Ma'aser to one's adult children whom he need not support. One may even give to his father, if he is poor. It seems that he permits even if the children could support themselves from another source, for also this is considered Tzedakah.

Beis Lechem Yehudah: One should have a special ledger for Ma'aser, from Rosh Hashanah until Erev Rosh Hashanah. One may deduct losses from profits as long as the money did not go to an Oni or box for Tzedakah or Ma'aser. After this, he cannot turn Kodesh into Chol. Shevus Yakov (2:86) is stringent; we may be lenient, especially since Maharam me'Rotenburg says that Ma'aser of money is only a custom. I.e. giving it to Aniyim is only a custom; the Sifri expounds it from "Tevu'as Zar'echa".

Mishbetzos Zahav (656:2): The She'altos says that we learn from Mar Ukva that one may give up to a third. The Rif and Rosh say only that one may give more than half. This requires investigation.