Gemara

(Mishnah): Amirah l'Gavoha (saying 'I give this to Hekdesh') is like Mesirah l'Hedyot (handing it over to a person).

Bava Kama 36b: A man hit Reuven. Rav Yosef ruled that he receives a half-Dinar. Reuven did not want it, and said 'give it to the poor'. He later retracted and asked 'give it to me. I will use it to make myself healthy.'

Rav Yosef: You cannot retract, for the poor already acquired it. Even though there are not poor people here, we are a Yad (power of acquisition) on behalf of poor people elsewhere.

Rosh Hashanah 6a: "B'Ficha" teaches that Bal Te'acher (the Isur to delay bringing Korbanos) applies to Tzedakah.

Rishonim

Rif (Bava Kama 18b): Rav Yosef meant 'Aniyim are no less than others. Even though there are not Aniyim here, we are Yad Aniyim to acquire for them through Ma'amad Sheloshtam (a special Kinyan in which David tells Levi, in front of Moshe, 'give the money you have of mine to Moshe'). If one said about money in his possession 'it is for Aniyim', they acquire immediately, due to "b'Ficha".

Rosh (Bava Kama 4:3): "B'Ficha" does not teach that his Dibur obligates him, for one cannot be Makdish or give to Tzedakah a Davar she'Lo Ba l'Olam (something not yet in the world).

Question: How could Rav Yosef say that he cannot retract? No one can claim the money! He can say that he will give to another Oni!

Answer (Rashba, brought in Teshuvas R. Betzalel Ashkenazi 14 DH Teshuvah Zu): Even though he can evade any Oni, b'Yedei Shamayim he is obligated. One cannot be Makdish what is not in his Reshus only regarding Hekdesh. A promise to give to Aniyim is a Neder, and he must fulfill it even if he did not have at the time. Sometimes we force him, e.g. if the Gabai Tzedakah of the city are there.

R. Betzalel Ashkenazi (DH v'Nireh): In the case with Rav Yosef, the Rashba obligates only due to the vow. If he were obligated due to a Kinyan (acquisition), he could not retract through Hataras Nedarim, for the poor already acquired. However, this is wrong. One can permit proper Hekdesh, even though Dibur (the words to Hekdesh) made a Kinyan! The Rashba himself (704) wrote that Dibur is not a Kinyan in every respect. It merely eliminates the need for Mesirah.

R. Betzalel Ashkenazi (ibid.): One can permit Hekdesh only because we hold that mistaken Hekdesh is invalid. This shows that a Pesach is required. The Rashba permits Hekdesh Aniyim through regret from the beginning. If it were totally as if it was given over, a Pesach would be required.

Rambam (Hilchos Mechirah 22:15): The law of Hekdesh, Aniyim and Nedarim is unlike that of regular people. If one said 'all offspring of my animal will be Hekdesh to Bedek ha'Bayis' or 'forbidden to me' or 'for Tzedakah', even though it does not become Kodesh because it is Lo Ba l'Olam, he must keep his word, due to "k'Chol ha'Yotzei mi'Piv Ya'aseh."

Ritva (28a DH Reshus): Amirah to Hekdesh Aniyim is like Mesirah l'Hedyot. It is like the appropriate Kinyan for the matter, e.g. writing and Mesirah for documents, and Chazakah for land. One must say all that must be said for people, e.g. 'acquire the lien in the document.' Only the Kinyan need not be done.

Poskim

Shulchan Aruch (CM 212:7): The law of Aniyim is unlike that of regular people.

Ketzos ha'Choshen (4): The Radvaz (4:134) and Maharitatz (230) say that one who vowed to give to Oni Ploni can give to another Oni. Hagahos Maimoniyos (Hilchos Zechiyah 11:4) says that one who vowed to particular Aniyim cannot change to other Aniyim because Amirah l'Gavoha k'Mesirah l'Hedyot, and these Aniyim already acquired. Most Poskim hold that Amirah l'Gavoha k'Mesirah l'Hedyot does not make a Kinyan to Aniyim.

Shulchan Aruch (CM 243:2): If one told a Shali'ach 'take this money to Ploni', he can retract as long as Ploni did not get it. If Ploni is an Oni, he cannot retract

Beis Yosef (DH u'Mah she'Chosav v'Im): The Tur says that one cannot retract a gift to an Oni, for Amirah is like Mesirah. This is wrong. We say so only regarding Hekdesh (Kidushin 28b), but an Oni is a commoner. Also, if Amirah were like Mesirah, he should have taught a bigger Chidush, that one cannot retract a promise to give to an Oni! However, the law is true. Rav Yosef didn't allow Reuven to retract because 'we are Yad Aniyim.'

Defense (R. Betzalel Ashkenazi 14 DH Nimtzeinu): The Beis Yosef holds like Rav Hai Gaon and the Ba'al ha'Ma'or, who say that Tzedakah is like a commoner, except that Bal Te'acher applies, due to 'b'Ficha'. Beis Din force him only verbally. Amirah is like Mesirah only for Hekdesh Mizbe'ach and Bedek ha'Bayis. The Rif, Tosfos and most Acharonim hold that Beis Din forces him. It is a Safek how we rule, therefore we do not force him. The Ran (Teshuvah 1) says that we do not say Safek Hekdesh Aniyim l'Chumra. Rather, it is like Safek between people, and we are lenient and do not make him pay. This is when the Makdish or his heirs are Muchzak. If Reuven commanded to give to Hekdesh and to Ploni, perhaps Hekdesh would have the upper hand in Metaltelim, for Amirah l'Gavoha is like Tefisah (seizure). Mahari Kolon (149) says that since all the Poskim we follow rule like the Rif, the one claimed from cannot say 'I hold like the other opinion.'

Rema (252:2): If Reuven vowed or swore to give a certain amount to Ploni and he died without giving, his heirs are exempt. There is no Mitzvah to fulfill the words of the deceased.

Ketzos ha'Choshen (290:3 DH uv'She'alos): The Panim Me'iros (2:135) says that this is because Amirah l'Gavoha k'Mesirah l'Hedyot applies only to Hekdesh, but not to Aniyim. The Tur (YD 258) allows permitting a vow to give Tzedakah, but not if he gave a debt to the Gabai through Ma'amad Sheloshtam (Bach, Shach 18). This is no proof. Hekdesh acquires totally like Mesirah, yet one can permit it! This is because Dibur can be Mevatel Dibur. He cannot permit it after Meshichah or Ma'amad Sheloshtam, for Dibur cannot be Mevatel Dibur.