Gemara

(R. Aba citing Rav): Leah asked Shimon to receive her Get, and he told this to her husband (Reuven). Reuven told him 'Holech, and give it to her.' Shimon is a Shali'ach of Reuven and of Leah. If Reuven died, she must do Chalitzah, but not Yibum (perhaps the divorce was valid).

Question: Here, Rav is unsure whether saying 'Holech' is like saying 'acquire for.' Elsewhere, he is sure that it is!

(Rav): If Reuven said 'Holech this money that I owe to Shimon', if the money does not reach Shimon, Reuven still owes the money. Reuven cannot tell the messenger 'I retract; return the money to me.'

Answer: There also, Rav is unsure. Regarding Safek Mamon (monetary laws), we follow the Chazakah (Reuven cannot retract). Safek Isur l'Chumra (we are stringent about Isurim such as divorce).

Nedarim 7a - Question: Do Yados work for Tzedakah? (If one said "this coin is for Tzedakah, and this", is the latter Tzedakah?)

This question is not resolved.

Chulin 134a (Mishnah): A convert must give Matanos (the foreleg, jaw and stomach) to a Kohen from his cow only if it was slaughtered before he converted. If we are unsure, he is exempt. The Kohen cannot take from him without proof.

Contradiction (Mishnah): If grain was found in an ant-hole in a place that was already harvested, what rests on top must be left for the poor;

R. Meir says, even what is inside must be left for the poor, because Safek Leket is Leket.

(Reish Lakish): "Ani va'Rash Hatzdiku" cannot mean to vindicate the poor - "v'Dal Lo Sehedar b'Rivo"! Rather, give to him what you were entitled to keep (if not for this verse).

Answer (Rava): The Chazakah is that Leket must be left from the grain, but (until the Nochri converts) the cow is exempt from Matanos Kehunah.

Question (Abaye - Mishnah): If a Nochri converted with a dough, he must take Chalah only if it was kneaded after he converted. If we are unsure, he must take.

Answer (Rava): One may not eat without taking Chalah, so Safek Isur l'Chumra. Matanos are a monetary privilege of the Kohen, and Safek Mamon l'Kula.

Rishonim

Rosh (Nedarim 1:3 and Ramban on Rif 1b): The Gemara did not resolve whether or not Yesh Yad l'Tzedakah, therefore we are stringent.

Rebuttal (Ran 7a DH ul'Inyan): Also the Rashba considers this Safek Isur l'Chumra. Chulin 134a says that Safek Mamon Aniyim is Safek Mamon, and we are lenient (for the giver)! Safek Leket is Leket because due to Chazakah (he was obligated). If it were Safek Isur, there would be no need to say this! Yoma (8b) understood that we are lenient about Ma'aser Oni of Demai (i.e. Safek) because ha'Motzi me'Chavero Alav ha'Re'ayah, but asked why we are lenient about Ma'aser Sheni. This is because Ma'aser Sheni is Safek Isur, and Ma'aser Oni is Safek Mamon!

Rosh (Bava Metzia 2:21): If a Chacham was in a field, and found an Aveidah that he would normally carry in a field but not in the city, since he is exempt from a full Hashavah, perhaps he is totally exempt. The Gemara never settled the question. Some are stringent, and obligate him. I exempt. He may not disgrace his honor, and be lenient about Safek Isur, in order to be stringent about Safek Mamon! If the Chacham wants to go beyond the letter of the law, he will pardon his money (and pay the owner), like R. Yishmael b'Rebbi Yosi (Bava Metzia 30b).

Poskim

Shulchan Aruch (YD 259:5): If one has coins and is unsure whether or not they are of Tzedakah, he must give them to Tzedakah.

Source (Mordechai Bava Basra 659, cited in Beis Yosef DH Kosav Od): Toras Kohanim and the Yerushalmi expound that Safek Leket, Shichchah and Pe'ah must be left for Aniyim. We may learn to Safek Tzedakah.

Rebuttal #1 (Shach 14): Rava (Chulin 134a) is stringent about Safek Leket due to Chazakah. Without a Chazakah, we are lenient! To answer Abaye, he said that Safek (Matanos) Aniyim is Safek Mamon!

Rebuttal #2 (Gra 15): Our Sugya favors Reish Lakish, who follows Chazakah. And even if the Halachah follows R. Yochanan, he exempts Safek Leket even of a Yisrael (against the Chazakah - Yerushalmi Pe'ah Sof Perek 4)!

Chasam Sofer (YD Sof 240): We are stringent about Safek Tzedakah, so it is like Safek Hekdesh. If one pledged to Hekdesh and does not remember how much, he brings an amount that he knows that he did not vow more than this.

Pischei Teshuvah (10): The Chasam Sofer brings other Rishonim who hold like the Rosh, Ramban and Rashba, that Safek Tzedakah is like Safek Isur. Only the Ran and Nimukei Yosef disagree; we do not follow them. The Oni is Muchzak. However, if the giver has a Vadai claim, e.g. a Shechiv Mera who recovered, he wins. This is even if one has a claim against him, e.g. he pledged to a particular Tzedakah. This is unlike the Rosh (Bava Basra 9:23).

Rema: However, if one was Makdish something with a Safek expression and he died, the heirs are Muchzakim. Hekdesh cannot take from them without a proof.

Teshuvas ha'Rashba (1:656, cited in Shach 14): If one was Makdish to Aniyim on Tenai and died, the heirs are Muchzakim. The Aniyim must prove that the Tenai was fulfilled, just like regarding Safek Matanos Kehunah. Safek Isur l'Chumra does not apply to Hekdesh Aniyim. Even regarding Hekdesh Bedek ha'Bayis, we follow Chezkas Mamon if it is a Safek whether Hekdesh took effect. We never resolved whether or not a Shechiv Mera can retract from Hekdesh, Hefker, or Hekdesh to Aniyim. The Rambam is lenient. Even the stringent opinion could agree if perhaps the Hekdesh never took effect.

Question (Taz 3): Above (258:2), the Rema (agreed with the Shulchan Aruch, who) ruled that Yesh Yad l'Tzedakah, i.e. because it is Safek Isur l'Chumra!

Answer #1 (Shach 14): The Rema holds that the Mordechai and the Rashba do not argue. (He rules like both of them.) Regarding heirs, this is not considered Safek Isur. However, the Rashba equates heirs and the Makdish. He brought proofs from Matanos and a Shechiv Mera who retracted! Also the Terumas ha'Deshen (Sof 2:73) and Rivash (160) equate them.

Answer #2 (Taz): The Rema is lenient if it was feasible for the Safek to be resolved. A Makdish is believed to explain what he meant. There is no way to resolve whether or not Yesh Yad l'Tzedakah, so the Rema is stringent.

Answer #3 (Chidush ha'Gershoni, in Otzar Meforshim in Friedman Shulchan Aruch): The Rema rules that Yesh Yad l'Tzedakah, like the Rambam, who says that 'if you will say that...', shows that the Halachah follows this opinion.

Shach (YD 177:68): If it is a Safek whether a case is Ribis Ketzutzah (usury mid'Oraisa) or Avak Ribis (mid'Rabanan), this is Safek Mamon. We are lenient, and ha'Motzi mi'Chavero Alav ha'Re'ayah. However, if most Poskim say that it is Ribis Ketzutzah, this is Safek Isur mid'Oraisa, and we follow the majority. Beis Din should threaten him that he transgresses, but if he does not heed them, they cannot take from him, for it is a Safek Din. We say so about Safek Matanos, even though there is a Mitzvas Aseh to give them.

Pischei Teshuvah (8): Bris Avraham (YD 49:1, in Hagahah) says that if it is a Safek Ribis Ketzutzah or Avak Ribis, we say Safek Isur l'Chumra, for in any case he should not have taken the money. He must return it. If it is a Safek Ribis Ketzutzah or Heter, he can say 'I hold like the opinion that permits.'