Gemara

(Beraisa - R. Chanina ben Tradyon): I once (collected Ma'os Purim), and mixed them wtih (regular) Tzedakah (and I gave it for Stam Tzedakah. When I realized my mistake) I donated that amount for Purim.

Bava Metzia 78b - Question: Where did R. Meir say that anyone who transgresses the owner's intent is a robber?

Answer (Beraisa): The collection for Purim must be given to the poor for Purim. Money collected for the poor of a city must be given to the poor of that city. We are not exacting. We slaughter calves for the poor to eat on Purim; the extra meat (is sold, the money) goes to Tzedakah.

R. Eliezer says, an Ani must use Purim money for the Purim meal. He may not use it to buy a shoe strap without stipulating in front of the people of the city. These are the words of R. Yakov, in the name of R. Meir.

R. Shimon ben Gamliel is lenient.

Rejection: There, the people gave the money only for the Purim meal. It belongs to the giver until it is used for the meal!

Megilah 7a (Rav Yosef - Beraisa): One must give two Mishlo'ach Manos (portions of food) to a friend, and two gifts to two poor people.

7b: Abaye bar Avin and R. Chanina bar Avin used to exchange their meals.

Rishonim

Rif and Rosh (Bava Metzia 48b and 6:9): The Halachah does not follow R. Meir, for he holds like his opinion (that one who transgresses the owner's wishes is a robber, and the Halachah does not follow him). We rule like R. Gamliel, who is lenient.

Hagahos Ashri: The Gabai may not rely on his judgment to separate for the needs of Purim. Rather, first he does the needs of Purim properly, and what remains is for Tzedakah.

Rambam (Hilchos Megilah 2:16): One must give to Aniyim on the day of Purim.

Rambam (17): It is better to increase gifts to Aniyim than to increase the meal or gifts to friends, for the greatest, most glorified Simchah is to gladden the poor, widows and orphans. One who does so acts like the Shechinah, which revives the spirit of the lowly.

Ran (Megilah 3b DH u'Mishlo'ach): One must give two portions for Mishlo'ach Manos, but it suffices to give one gift to each Ani, for it is considered great in the eyes of the Ani.

Kaf ha'Chayim (694:8): The Magen Avraham (695:12) explains that it is because one should not give all his Tzedakah to one Ani.

Hagahos Ashri (Megilah 1:7): If one intended to distribute coins on Purim, he may not change them. He must give those coins.

Beis Yosef (OC 694 DH Kasuv): He holds that like some Poskim (brought in YD 258) say, that thought to give Tzedakah is binding.

Tosfos (Bava Metzia 78b DH Rebbi): The first Tana discusses only the Gabai, and teaches that he may not change (to another Tzedakah). R. Eliezer teaches that the Ani who received the Tzedakah may not change it.

Mordechai (Bava Basra 491): Since the Gabai collected for Purim, he cannot change it to another Tzedakah, but the Ani may change it. The people of the city can change it for Aniyim of above city.

Poskim

Shulchan Aruch (OC 694:1): One must give at least two gifts to two Aniyim.

Taz (1): Even an Ani who is supported by Tzedakah must give, unlike normal Tzedakah, which he must give only once a year to fulfill the Mitzvah of Tzedakah.

Magen Avraham (1): He must give his own money. He may not give Ma'aser money.

Mishnah Berurah (1): Both men and women must give. Many obligate even an Ani supported from Tzedakah to give from what he received.

Mishnah Berurah (2): One may give food or money. The Pri Megadim did not know what the Shi'ur Is. The Ritva says that even a Perutah is considered a gift. One who swapped his meal with another fulfilled giving a gift. Therefore, an Ani supported by Tzedakah should give gifts to two Aniyim, and they will return to him. L'Chatchilah, one must give an Ani something he can benefit from on Purim, either food or money that he can spend on Purim.

Kaf ha'Chayim (7): Some say that even a Perutah is considered a gift. Zera Yakov says that it must be the value of a Seudah, just like Mishlo'ach Manos. Abaye bar Avin and R. Chanina bar Avin used to exchange their meals. This shows that one is not Yotzei with less than a meal. The same applies to gifts to Aniyim. Therefore, one must give money to buy (the volume of) three eggs of food, or three eggs of bread. Even if one gives a Perutah to each of 20,000 Aniyim, he was not Yotzei. Zera Yakov concluded that he is not so sure about this. One should be stringent to fulfill all opinions for gifts to two Aniyim. (One who gives to more than two may give a Perutah to the additional Aniyim.)

Ashrei ha'Ish (3:46:13, citing ha'Gaon R. Y.S. Elyashiv, Shlita): Perhaps the Ritva would agree that nowadays, a Perutah is not enough, for it is insignificant. One must give something that an Ani will not be embarrassed to take, and it will make him happy. It is better to give two significant gifts to two Aniyim than to give small gifts to many Aniyim. In 5762 (2002), a gift of 50 Shekalim (about 13$) would fulfill the Mitzvah beautifully.

Mishnah Berurah (3): Letter of the law, one is Yotzei with two gifts. It is better to increase gifts to Aniyim than to increase the meal or gifts to friends. What one adds above two gifts may be from Ma'aser.

Kaf ha'Chayim (4): The Rashba says that we do not bless on this, for perhaps the Ani will not accept it. However, one should say 'l'Shem Yichud... I come to fulfill the Mitzvah...'

Kaf ha'Chayim (10): If one gave to an Ani and his wife, this is like giving to two Aniyim.

Kaf ha'Chayim (12): One who gave to a minor is Yotzei, if the minor understands that he receives Ma'os Purim. One who gave Mishlo'ach Manos to a minor was not Yotzei , for it says "l'Re'ehu (to his colleague)."

Kaf ha'Chayim (15): The Ba'al ha'Ma'or says that one should not give Ma'os Purim before Purim, lest the Ani consume it before Purim. We infer that if one sent Ma'os Purim before Purim and the Ani received it on Purim, he was Yotzei. The same applies to Mishlo'ach Manos.

Kaf ha'Chayim (18): The Ari Zal says that gifts to Aniyim and Mishlo'ach Manos should be done after Kri'as ha'Megilah.

Shulchan Aruch (2): One may not change Ma'os Purim to a different Tzedakah.

Beis Yosef (DH Ma'os): Rashi explains that 'the collection for Purim' is money collected to give to the poor for the meal of Purim. All of it must be given to Aniyim on Purim. We are not exacting to say that this is too much for Purim, and the excess should go to the (regular) Tzedakah fund.

Gra (3): The Rambam explains that we are not exacting, i.e. we give to anyone who requests.

Beis Yosef (DH v'Yesh): Why does the Tur rule like the Beraisa that says that an Ani may not use Ma'os Purim to buy a shoe strap? The Rif and Rosh rule like R. Gamliel, who is lenient! Also the Rambam omitted this law that an Ani may not use Ma'os Purim for other things. In R. Yerucham's text of the Beraisa, R. Meir said his law in the name of R. Akiva. He says that many rule like R. Akiva against his colleague, unlike the Rif, but the Rif's opinion is primary.

Mishnah Berurah (6): We use all the money for calves. What the poor do not eat on Purim we sell, and the money goes to Tzedakah.

Rema: This refers only to the Gabai, but the Ani may change it.

Magen Avraham (5): The Tur and Rema connote that even the people of the city cannot change it. The Mordechai explicitly says that they can. If so, it is like regular Tzedakah.

Gra (6): Tosfos rules like the first Tana, that the Gabai may not change. The Rif and Rosh rule like R. Shimon ben Gamliel who is lenient, unlike the first Tana, for R. Meir holds like his opinion that anyone who transgresses the owner's intent is a robber, and the Halachah does not follow him in the Mishnayos about a borrowed animal (78a) or a dyer who deviated (Bava Kama 102a). Our text is difficult, for it begins with R. Eliezer, and concludes with R. Yakov in the name of R. Meir. The Rif's text is correct. The Yerushalmi asked, may we infer that other Tzedakah money may be changed?! Rather, other money may be changed until it is give to the Gabai.