Gemara

(R. Yochanan and Reish Lakish): If Reuven swore 'I will eat this loaf today' and he did not eat it, he is not lashed;

(R. Yochanan): He is not lashed because it is a Lav she'Ein Bo Ma'aseh (one transgresses through inaction).

He does not exempt due to Hasra'as Safek (perhaps he will eat it later, and not transgress). He holds that it is valid warning.

(Reish Lakish): He is not lashed because Hasra'as Safek is invalid Hasra'ah;

Nedarim 59a (R. Aba): Since one can permit his vow, it is like a Davar she'Yesh Lo Matirin (it can be permitted without Bitul), which is not Batel in a majority of Heter.

Question: One can permit a declaration of Terumah, yet it can be Batel!

(Mishnah): If one Se'ah of Tamei Terumah fell into less than 100 Sa'im of Chulin, it must all rot.

Inference: Had it fallen into 100 Sa'im, the Terumah would be Batel!

Answer: Vows are different, for it is a Mitzvah to permit them;

(R. Nasan): One who fulfills a vow (does not permit it) is like one who offered a Korban on a private altar [when this is forbidden)].

There is no Mitzvah to permit a declaration of Terumah.

Rishonim

Teshuvas Maimoniyos (Sof Sefer Zera'im, citing Sefer ha'Mitzvos, brought in Taz 2): Through She'elah, Kedushas Chalah is uprooted retroactively. The Shulchan Aruch (YD 331:48) brings so regarding Terumah and Ma'aser that one regretted.

Question (Taz YD 323:2): A Mishnah teaches if Tamei Terumah fell into less than 100 parts of Chulin, we leave it all to rot. I.e. there is no solution. If there were a solution, the Mishnah should teach it, like it taught solutions to other cases. This is clear from the Rambam, who wrote (Perush ha'Mishnayos Terumos 5:1) that he leaves them to rot. Rashi in Nedarim (59a DH Tirakev) says that there is no solution. He should be Sho'el on the Terumah to permit it to a Yisrael, and separate more Terumah!

Suggestion: It is a case in which She'elah is not possible.

Rejection (Taz): In Nedarim, we derive that if it fell into 100, it is Batel. The Gemara asked that it is Davar she'Yesh Lo Matirim, since one can be Sho'el, like it said about a Neder! It answered that a Neder is different, for it is a Mitzvah to permit Nedarim, but not to permit Terumah. This shows that She'elah is possible. Why do we say to be Sho'el to permit Chalah, but not Terumah? Also, if She'elah helps to prevent a loss due to a mixture of Chalah or Terumah, how can there be Kares for Shechutei Chutz or Misah b'Yedei Shamayim for Terumah? Surely he will be Sho'el due to fear of punishment!

Tosfos (Gitin 33a DH v'Afke'inhu): R. Shmuel asked, how can we kill a married woman who was Mezanah? Perhaps her husband will send a Get to her and be Mevatel it [not in front of her or the Shali'ach], and Chachamim uprooted the Kidushin, so retroactively she was never married! It is Hasra'as Safek! R. Tam says that we follow the majority. Most men do not divorce their wives, and when they send a Get, they are not Mevatel it. Also, we leave her in her Chazakah. Now she is married. If not, why is a Nazir lashed for drinking wine or becoming Tamei Mes? Perhaps he will be Sho'el on his Nezirus!

Ran (Nedarim 58a DH v'Chol): Even though one can be Sho'el about Terumah and Chalah, since there is no Mitzvah to do so, it is not called Davar she'Yesh Lo Matirim.

Rashbam (Bava Basra 120b DH she'Yesh): Just like She'elah applies to Nedarim, it applies to all Hekdesh. If one slaughtered Kodshim outside the Mikdash and was Chayav Kares, and later went to a Chacham to permit his Hekdesh and found a Pesach or regret and uprooted the vow from the beginning, and made it like a mistaken vow, retroactively it was never Hekdesh, and he is exempt from Kares. The same applies to anything Kadosh due to words, such as Terumah or Ma'aser. He can be Sho'el and they return to Tevel, like it says in Nedarim.

Poskim

Shulchan Aruch (YD 323:1): Chalah of Chutz la'Aretz does not forbid a mixture, even if it became mixed with its own volume.

Rema: Some disagree, and hold that it forbids its mixture until 101 parts [of mixture] Min b'Mino, and Min b'Eino Mino until 60.

Bach (324:18 DH Kasav): Mahariyo says that if Chalah was not separated from a dough and it was cooked, one separates the proper Shi'ur from what remains from the dough. It works retroactively. We use Breirah for mid'Rabanan laws. Also, one may separate Chalah of Chutz la'Aretz after eating (some) of the bread. We require 101 for Bitul only if it became mixed after it was separated. The Beis Yosef says so.

Shach (5): Even this opinion is stringent only if Chalah was separated, and became mixed. The Rambam permits l'Chatchilah to mix it with its own volume of Chulin to be Mevatel it!

Rema (ibid.): If the Chalah became mixed with the dough, and there are not 101 parts for Bitul, if he did not eat [any of] the dough, he is Sho'el a Chacham, and he permits it.

Taz (2): If not for Semag, I would answer that She'elah helps for Terumah and Ma'aser when he regrets the designation itself, but not due to a mixture. Every Pesach (reason for regret) must uproot the vow itself, that if the vow will continue, he will surely be harmed. Here, he could have made Terumah or Chalah and guarded it from becoming mixed! The Ran and Rashbam explicitly say so.

Taz: The Rashbam requires a Pesach of regret about the vow itself, but not because later it became mixed or he slaughtered Kodshim b'Chutz. We say that a Neder is Davar she'Yesh Lo Matirim, for perhaps he will find a Pesach of regret about the vow itself. The Gemara concluded that it is a Mitzvah to permit vows, therefore it is as if he already permitted it. This does not apply to Terumah, so it is not Davar she'Yesh Lo Matirim. This is why when Chalah became mixed with dough, it must rot. The Beis Yosef (619) brings from R. Tam that Kol Nidrei does not help for vows that one transgressed, for a Chacham permits only through regret. Regret that he transgressed them does not suffice! The Rosh argues, and says that surely anyone who transgressed regrets the vow from the beginning, in order to be saved from the punishment. There is different. Since he transgressed, he sees that he will be punished, and regrets that he vowed at all. Fear of punishment is on him from the moment he vowed, since he is the type who can transgress his vow. Terumah and Chalah are not prone to become mixed. It just happened to occur. He does not uproot the vow from the beginning. Really, the Rosh is astounding. If so, how can a Nazir be lashed, or someone receive Misah [b'Yedei Shamayim] for eating Terumah? (I do not understand the question from Terumah. The Rosh agrees that he does not regret his separation. He was commanded to do so!) They will go to a Chacham to permit the vow! R. Shmuel asked that this is Hasra'as Safek. If we permit She'elah due to fear of punishment, surely he will be Sho'el! One may not rely on this Heter to be Sho'el based on regret that it became mixed. Perhaps even Semag discusses only when he has regret about the Chalah itself.

Rebuttal (Nekudas ha'Kesef): The Mishnah teaches the law, that they must rot. It did not teach other solutions. It discusses one who does not want to permit his declaration. We could have said that it is Davar she'Yesh Lo Matirim, therefore we needed to say that there is no Mitzvah to permit it. We permit Hekdesh only in pressed circumstances. One is Chayav Kares if he does not permit the Hekdesh. A Nazir who does not permit his vow is lashed.

Birkei Yosef (1): Nekudas ha'Kesef refuted the Taz. The Beis Yosef said so in Bedek ha'Bayis. (Words are missing from the version printed amidst the Beis Yosef.) This is the custom.

Shach (6): Darchei Moshe says that if the Chalah was already eaten, one cannot be Sho'el. Here, many Poskim say that it is Batel in a majority of Chulin, so even without She'elah he did not transgress. [This is unlike one who ate the Chalah, for] all forbid Tevel.

Shach (7, based on Be'er Heitev 3): We do not say that even 101 are forbidden due to Davar she'Yesh Lo Matirim. There is no Mitzvah to be Sho'el about [Chalah, which resembles] Hekdesh, so it is not considered Davar she'Yesh Lo Matirim (Ran).

Pischei Teshuvah (2): She'alas Ya'avetz (2:99) says that it is Batel only if it became mixed with another dough. The Rema did not say that it became mixed with its dough, for then it is not Batel. It returns to be Tevel. One separates again from the dough. The Chasam Sofer (319) says that one may surely rely on this along with She'elah. However, if it fell into another dough, we permit through She'elah only for a great need.

Rema (ibid.): This is because one may be Sho'el about Hekdeshos. He then separates another Chalah.

Birkei Yosef (1): One should not bless when separating the second time, for the Berachah is not Me'akev, and one should be slightly concerned for the Taz.