Baba Metziah 20a - Eidav b'chosamav zachin

The gemara says that when a woman says that she wrote a receipt to her husband that she received the kesuba, you can return it to him. The reason is that even if she didn't give the receipt to her husband on the date written in it, which can lead to harming someone who may have purchased her kesubah, the husband is automatically zocheh in the receipt when he receives it retroactively from the date written in it. This is based on Abaye's approach of עדיו בחותמיו זכין לו. Where does this idea of עדיו בחותמיו זכין לו come from?

Tosafos explains that the there was a takanas chachamim that the beneficiary of a contract should be zocheh in the document from the day on which it is dated, regardless of when it is delivered, so that the witnesses won't have to ensure that it is delivered on the day that it was written. The maharsha suggests that this is only necessary according to r' elazar who normally says that the eidei mesirah make the contract effective, so we need a special takanas chachamim. But, according to R' Meir who holds that the eidei chasima make the contract effective m'doraysa, of course it will be binding from the day on which it is dated (maharam shif challenges this).

Tosafos continues to question what would be with a contract that effects issur, such as a shtar to free a slave that would permit him to a bas yisroel. In a situation where the shtar was dated in nissan but only delivered in Tishrei, would we consider a bas yisroel who had relations with the "eved" between nissan and Tishrei to be passul l'kehuna since he was still an eved at the time, or can we use the eidim to make the shtar effective retroactively from nissan. Tosafos says "maybe even by a shtar that impacts issur can be effective from the time it was signed, even though the rationale for the takana isn't applicable - לא פלוג רבנן ויש כח ביד חכמים לעקור דבר מן התורה. The mishneh l'melech (zechiya u'matana 9:10) points out that Tosafos holds that even when a takanas chachamim is going to have an impact on issur, chazal would still maintain their takana.