Gemara

"V'Im Mach Hu me'Erkecha" teaches that Heseg Yad applies only if he was Mach (poor) from the beginning (when he vowed the Erech) until the end.

Question: If so, "v'Hu Ed" should teach that a witness must be Kosher (to testify) from the beginning (when he saw the testimony) until the end (when he testifies)!

(Beraisa): The general rule is, if Reuven was Pasul at the beginning (when he saw the testimony) or the end (when he wants to testify), he cannot testify. If he was Kosher at the beginning and at the end, he can testify.

Answer: Testimony is different. "Oh Ra'ah... Im Lo Yagid" teaches that it depends only on being Kosher at the time of seeing and testifying.

Kesuvos 20b: Rav Ashi knew testimony for Rav Kahana, but forgot it, even after Rav Kahana tried reminding him of it. Later, he remembered and testified for him.

Rav Ashi: I put my mind to it and remembered.

Shevuos 31b (Mishnah): If Reuven made witnesses swear Shevu'as ha'Edus (an oath that they do not know testimony), five times in Beis Din (and they really knew), they are liable only once.

R. Shimon says, this is because once they swear that they do not know, they can no longer testify.

32a (Beraisa): "L'Achas" teaches that they are liable for every oath.

Inference (Abaye): This must be outside of Beis Din, for the Mishnah teaches that in Beis Din, they are liable only once!

Rishonim

Rambam (Hilchos Edus 3:5): If a witness' testimony was researched in Beis Din, he cannot retract, e.g. to say 'I erred', 'I was Shogeg, and I remembered that it was not so', or 'I was afraid', we do not heed him, even if he gave a reason.

Rosh (Teshuvah 59:1): If a witness said 'I do not remember how many years he lived there', and later said that he remembered, he is not believed. What he did not testify at the time, he cannot testify later. This is why one is liable for Shevu'as ha'Edus in Beis Din only the first time, for then he could have retracted.

Ran (Shevuos 14b DH Mipnei): If they initially denied in Beis Din, they cannot retract, for they already testified that they do not know. Therefore, even if they denied only at the end, all the oaths after the first were l'Vatalah. Why are they liable? Once they denied, they cannot testify! The Gemara does not connote that it was Toch Kedei Dibur. We can say that they denied outside of Beis Din, therefore they can retract. However, the Gemara does not connote like this. I prefer to say that Im Lo Yagid, from which we expound that one cannot change testimony, is written after "v'Shom'ah Kol Alah (he heard an oath, i.e. and accepted it)." It is normal for witnesses to evade testifying. Even though they initially said that they do not know, if afterwards they said that they know, this is not called retraction. The Torah considers denial to be an Aveirah only after an oath. Even though witnesses' signatures is considered testimony that Beis Din investigated, and they cannot retract (Kesuvos 18b), that is because their testimony already finished. Oral testimony finishes only after the claimant makes them swear. Also, switching from 'we do not know' to 'we know' is not called retraction. At first they did not testify at all, and now they testify! We say that they cannot retract and admit (that they know) not because this is retraction, rather, because they cannot make themselves Resha'im, since initially they swore.

Hagahos Ashri (Shevuos 4:15, citing Or Zaru'a): When they make a Cherem in the Beis ha'Keneses for testimony about a matter, surely one who said 'I do not know' cannot retract and say 'I forgot, and now I remembered.' Rather, even if one was silent, and later wants to testify, we do not use his testimony to make someone pay due to Safek, for perhaps his silence was denial. After denial in Beis Din, one cannot testify. This is if they were in the Beis ha'Keneses and heard the Cherem, and it was made 'on everyone in the Beis ha'Keneses.'

Mordechai (Shevuos 763, from Sefer ha'Chachmah): A case occurred in which a Tzibur imposed a Cherem, that anyone who knows must testify before leaving the Chatzer of the Beis ha'Keneses. They left, and afterwards witnesses who had been in the Beis ha'Keneses said that it had not crossed their minds to testify, and now they remember and come to testify. We accept their testimony. We find that Rav Ashi knew testimony for Rav Kahana. Initially, he did not remember it. Later, he remembered and testified. Why is this unlike Shevu'as ha'Edus? One is liable only the first time, for they cannot retract and testify! We can say that there is different, for they totally denied. Once he testified, he cannot retract.

Tosfos (Bava Kama 56a DH Pshita): Granted, if witnesses swore that they do not know testimony, they cannot retract. (If they did not swear, why can't they retract?) I answer that the Korban is only if they swore falsely about not knowing testimony, but they bear sin even without swearing. This is why witnesses are exempt for Shevu'as ha'Edus if they previously denied in Beis Din, but if they denied outside of Beis Din, they can still testify.

Beis Yosef (CM 29 DH ume'Ha): The Ran holds that even if witnesses did not give a reason why they initially said that they do not know, we accept their testimony. However, the Mordechai and Tosfos hold that Im Lo Yagid applies after a denial in Beis Din. However, perhaps they distinguish when the witnesses give a reason why they retract.

Poskim

Rema (CM 29:1): When they make a Cherem in the Beis ha'Keneses for testimony, that they must testify before leaving the Beis ha'Keneses, and witnesses say 'it did not cross our minds to testify. Now we remember', they can testify, since initially they were silent. They did not say 'we do not know.'

Beis Yosef (ibid.): The Tur and Rambam connotes that after he testified he cannot retract, but if they asked him about the testimony and he said 'I do not know', he can retract if he gives a reason. We say 'once he testified, he cannot retract.' This connotes once he testified, he cannot retract and say (differently), but saying 'I do not know' is not called Magid (testifying). Since he gives a reason why he initially said that he do not know, he is believed. Also, when he testified he cannot retract, for he makes himself a Rasha for not being careful not to testify falsely. If he said 'I do not know' due to fear or due to another reason, he does not make himself a Rasha. He did not transgress anything! Even though he bears sin "if he will not testify", he was passive. Rashi (Bava Metzia 39b DH Tarti) connotes similarly regarding the brother of Mar bar Isak. Mari was powerful. People would do one (improper) matter, but not two. I.e. they would refrain from testifying (due to fear), but they would not even testify falsely. This shows that even when we do not suspect lest witnesses testify falsely, we suspected lest they refrain from testifying due to fear. Also, in Shevuos we say that if witnesses swore five times in Beis Din that they do not know testimony, they are liable only once. Why doesn't the Mishnah teach that if he claimed from them (without an oath) five times that they testify for him, and they denied, and in the end they swore, they are exempt, because when they swore the first time they could not retract and admit that they know testimony?

SMA (8): If they said in Beis Din 'we do not remember now', they can retract and testify. If they said Stam 'we do not remember', they cannot retract and testify.

Question (Taz): In Kesuvos, Rav Ashi said Stam that he does not know, and later he retracted and testified!

Answer (Nesivos ha'Mishpat Bi'urim 3): The Beis Yosef explicitly says that Rav Ashi gave an excuse (now I put it to my heart, and remembered) why he did not testify beforehand. Also the Rema says so.

Pischei Teshuvah (4): Pnei Moshe (1:17) says that if he initially said 'I do not remember', he can retract and say 'I remembered.'

Rema (ibid.): Even if they said 'we do not know', and later they gave an excuse why they said so, they retract and testify, since they did not say opposite to what they say now.

Question (Tumim 4): Abaye proved that "l'Achas" must discuss an oath outside of Beis Din. According to the SMA, perhaps the verse teaches about one who swore repeatedly in Beis Din "I do not remember now", for he can retract!

Question (Tumim 5): The Rema says that with an excuse, one can retract and testify. It seems that this is even if there was a Cherem. The Mishnah teaches that in Beis Din they are liable only once, for they cannot retract. They can retract through an excuse!

Answer (to both questions - Nesivos ha'Mishpat ibid.): Rashi (31b DH Ho'il) said 'since they cannot retract if they initially denied, therefore even though they did not deny until the end, all Shevuos after the first were l'Vatalah. If their initially silence is denial, it was improper to make them swear. If their initially silence is not denial, there is already a Shevu'ah on them, so another Shevu'ah does nothing. Rather, we must say that "l'Achas" discusses a Shevu'ah outside of Beis Din and denial in Beis Din. Rashi must mean that since the Torah taught that one who hears Shevu'as ha'Edus is liable as if he said it, unlike other oaths, and the primary oath is 'testify for me', one is liable only if after his denial he cannot testify. As long as he can testify, he must testify due to the first oath. Imposing another oath is like a Shevu'ah on a Shevu'ah (which does not take effect). However, outside of Beis Din silence is not denial. It is not like an oath on an oath. Even though the first oath obligates testifying, since they can admit and testify and (if so) they did not transgress the first oath, they are liable for each. Therefore, even if one can testify through an excuse, or if he initially said 'I do not remember now', he is not liable for later oaths, since the first oath obligates him to testify, and it is an oath on an oath when he can retract. Rather, the verse must discuss outside of Beis Din. This is even if it is not denial, e.g. he was silent or said 'I do not remember now.' In Beis Din we cannot distinguish the oaths if he denied. R. Shimon taught that it is because he cannot retract, i.e. in a case of denial. Further, if his excuse is only for the testimony, but not for swearing falsely, he is Pasul for testimony because he swore falsely! If he has an excuse even for the oath, e.g. he was mortally threatened, he can testify in spite of the first oath.

SMA (10): The Mechaber (above in this Sa'if) said that an excuse does not help, i.e. to say the opposite of what they said before.

Pischei Teshuvah (6): The Chasam Sofer (CM 34 DH Achar) says that if witnesses initially said 'Ploni owes Reuven', they cannot retract to say 'we had forgotten that he paid him', since they testified that he owes. However, if they initially said 'Ploni lent to Reuven', they can retract to say 'we had forgotten that he paid that loan', since this does not contradict their initial words. This requires investigation.

Chidushei ha'Rim (Shevuos 31b DH v'Nir'eh): Even if the first Shevu'ah was true, i.e. he did not remember testimony at the time, perhaps later he remembered! Why is it a retraction to say so? It seems that just like a witness who was Pasul and became Kosher may not testify, also one who was Kosher and became Pasul. In the middle is different. Poskim say that if one testified when he was Pasul, he cannot testify again when he becomes Kosher, for a person is prone to defend what he said (and not admit that it was false). The Gemara asked that a witness who was Pasul in the middle should be disqualified, and answered that the Torah is particular only when he is called a witness, i.e. at the time he saw and when he testifies. This is why he cannot testify again when he returns to Kashrus. Once he was called a Pasul witness, his testimony is Batel. This is why saying 'I do not know' in Beis Din is called testimony, and he cannot retract it, even if he remembers later. When he testified he was called a witness to nullify his testimony. Therefore, he cannot become a witness again.