Gemara

(Abaye): If Shimon testified and was Huzam, all testimony that he gave or will give after his fabricated testimony are invalid;

(Rava): Only testimony he gives after he was Huzam is invalid.

Version #1: Rava disqualifies only from now and onwards because the law of Edim Zomemim is a Chidush (exception). Why should we believe the latter witnesses (Mezimim)? Perhaps they are lying! It is a Chidush that in this trial (and surely, for future testimonies) that we assume that the first pair lied. We have no source to consider them liars before this!

32a - Question: Monetary cases do not require Drishah v'Chakirah (interrogation of testimony to enable Hazamah, i.e. asking the precise time and place)!

(Beraisa): If Yo'av and Michah were signed on a document dated 'Nisan 1, in Shemitah (in the place Ploni)', and Reuven and David said 'they were with us that day elsewhere (too far to get to Ploni the same day)', the document and its witnesses are Kesherim. We assume that it was postdated.

Question: Why didn't the questioner ask from a Mishnah?

(Mishnah): Postdated loan documents are Kesherim.

If we require Drishah v'Chakirah, why are they Kesherim?

Answer: The Beraisa is a bigger Chidush. Even though it is unlikely that one would postdate a document and give a date in Shemitah, since it looks suspicious (people usually refrain from lending in Shemitah, lest the loan be cancelled), we assume that this was the case. Shemitah does not cancel loans until the end of Shemitah, therefore the document is valid.

Answer (to the first Question - R. Chanina): Our Mishnah teaches that the Torah requires Drishah v'Chakirah for monetary cases. Chachamim enacted that monetary cases do not require Drishah v'Chakirah.

Rishonim

Rif (Bava Kama 28b): The only way to be Mezim Edei Chasimah (witnesses who signed a document) is if it is dated (e.g.) Nisan 10 in place Ploni, and others testify that that day they were too far away to reach Ploni that day. This is only if they testified in Beis Din that they signed the document on the date written. If not, a Beraisa teaches that they and the document are Kesherim, for we assume that it was postdated.

Ba'al ha'Ma'or (Kesuvos Sof 8b): Witnesses who signed the document cannot be Huzam, for perhaps they postdated it. Also, an Ed Zomem pays money only for verbal testimony.

Ran (Kesuvos 12b DH d'Kivan): He holds that even if the document says that it was written on the day it says, Hazamah does not apply. Hazamah is a Chidush; we can apply it only where the Torah said. The Rif says that Hazamah applies when the document says that it was written on the day it says. His opinion is primary.

Bach (CM 34:13): The Ba'al ha'Ma'or meant only that we punish the witnesses Ka'asher Zomam only for verbal testimony, but he agrees that the witnesses are disqualified. This is no less than Hazamah not in front of the witnesses. It is not Hazamah, but the witnesses are contradicted, their testimony is Batel and they are Pasul for all testimony (38:13).

Shach (34:13 and 38:2): The Bach is correct, but his reasoning is wrong. The same applies even if the Hazamah was not in front of the witnesses without signed! Rather, it is a Chidush that we punish Edim Zomemim even though they did not do an act. The Ba'al ha'Ma'or does not extend this. He said 'paying money due to Hazamah applies only to vt.' The Ran erred. Really, the Ba'al ha'Ma'or agrees with the Rif. If not, Sanhedrin 32a refutes the Ba'al ha'Ma'or! It says that mid'Oraisa, Drishah v'Chakirah is needed even for documents, i.e. to enable Hazamah. Surely, if the Ba'al ha'Ma'or argued with the Rif, he would have explicitly challenged him, and the Ramban would have defended the Rif. Perhaps everyone agrees to the Ba'al ha'Ma'or's law. The Ba'al ha'Ma'or said 'Hazamah does not apply to witnesses who signed a document, for we can say that it was postdated.' I.e. normally Hazamah does not apply, except for a contrived case it is they wrote 'this document was written on the date.' Indeed, in such a case Hazamah applies. Milchamos Hash-m did not challenge the Ba'al ha'Ma'or, for he does not argue with the Rif.

Divrei Chayim (Hashmatos 2:31): The Bach means that there are two elements in Hazamah. One is the testimony itself, i.e. does the borrower (or Shomer...) really owe. Hazamah not in front of the witnesses is contradiction because it is in front of the relevant parties, e.g. the lender and borrower. Disqualification of the witnesses is a separate matter. The Bach's comparison is good, for written testimony is Pasul. We rule based on a signed document, for it is like testimony investigated and accepted in Beis Din. It is as if they testified orally in Beis Din at the time they signed, or because it was written according to the borrower's command. This applies only to a Kosher document. Mid'Oraisa, we are not concerned lest it was forged. (The rest is missing. A Hagahah concludes: if the witnesses were Huzam we cannot say that their testimony was accepted. Therefore, it is written testimony, which is Pasul. Therefore, Hazamah disqualifies the document, but does not disqualify the witnesses even if they are in Beis Din, for even without Hazamah their testimony is Pasul, because it is written.)

Rambam (Hilchos Edus 19:3): Edei Chasimah are Huzam only if they said in Beis Din that they wrote the document on the date written and did not postdate it. If not, even if the document is dated Nisan 1 in Yerushalayim, and others testify 'the witnesses were with us in Bavel that day', the witnesses and the document are Kesherim. Perhaps the witnesses were in Yerushalayim in Adar, and postdated the document.

Rosh (Bava Kama 7:6): Abaye disqualifies Edim Zomemim from when they testified. E.g. they signed a document dated Nisan 1, and they testified in Beis Din that they wrote and signed the document on the date written, and others testify that that day they were in a different place.

Poskim

Shulchan Aruch (CM 34: 9): Edei Chasimah are Huzam only if they said in Beis Din that they wrote the document on the date written and did not postdate it.

Shach (12): The same applies if it says in the document that it was written on the date it says, like the Ran says.