Daf Yomi Ketubot 85a: Tear or Impair?

From my translation of the Rif:

ההיא איתתא דמחייבה שבועה בי דינא דרבא
אמרה בת רב חסדא ידענא בה דחשידא אשבועתא
אפכה רבא אשכנגדה

זמנין הוה יתיב רב פפא ורב אדא בר מתנא קמיה אייתו ההוא שטרא לקמיה
אמר ליה רב פפא ידענא ביה דשטרא פריעא הוא
אמר ליה איכא איניש אחרינא בהדי מר
אמר ליה רב אדא בר מתנא ולא יהא ר"פ כבת רב חסדא
אמר ליה בת רב חסדא קים לי בגוה ומר לא קים לי בגויה
אמר רב פפא השתא דאמור רבנן קים לי בגויה מלתא היא כגון אבא בר מר דקים לי בגויה קרענא שטרא אפומיה
קרענא סלקא דעתך אלא מרענא שטרא אפומיה
ולא מפרע מידי מיניה אלא בשבועה אי נמי בשודא דדייני יהבינא לההוא דלא איתרע שטריה

There was a woman who was obligated in an oath in the courthouse of Rava. The daughter of Rav Chisda {=Rava's wife} said, "I know about her that she is suspect in her oaths." Rava flipped over the oath to the one opposite her {in judgment}.

Another time, Rav Papa and Rav Ada bar Matna sat before him {=Rava}. A certain bond was brought before him.
Rav Pappa said to him: I know about this that this is a paid-off bond.
He {=Rava} said to him: Is there another man together with master {such that we will have two witnesses}?
Rav Ada bar Matna said to him: And is not Rav Pappa like the daughter of Rav Chisda?
He said to him: The daughter of Rav Chisda, I know I can rely on. But Master, I do not know if I can rely on.
Rav Pappa said: Now that the Sages say "I can rely upon him" is has validity, someone such as Abba bar Mar, upon whom I can rely, I would tear up a bond on his say-so.
Tear-up, does it arise in your mind?
Rather, "impair a bond on his say-so."
{Rif explains:} and not allow payment of anything from it without an oath. Alternatively, with the discretion of the judges we give it to he whose bond is not impaired.

This is clear emendation on the part of the setama digmara. The word קרענא is emended to מרענא, changing the ק to a מ, and the meaning from "tear" to "impair." Yet tearing is a physical act one can do to a document, and furthermore lectio difficilior applies, where they explicitly change the text to something else because of a problematic implication. Perhaps "tear" was meant figuratively. But maybe it was meant literally. Just as the daughter of Rav Chisda was trusted to prevent swearing, Abba bar Mar is trusted to say that the bond has been paid off. If he is actually trusted on this fact about this particular bond, why should we allow the holder of the bond (the lender) to swear? This would be treating him as a single witness, but the contrast is to Rav Pappa, where Rava required another person to say the same. And if another person said the same, the bond would have been torn up. Thus, the first suggestion of the Rif certainly appears problematic. Which is why a second explanation is offered. That it is based on shuda dedayni. But if so, and the judge, as here, can decide to not let him collect with this shtar, then why not rip it up?

Meanwhile, Tosafot (pictured to the right, from Ketubot 85a), based an Rabbenu Chananel, says the same, first explanation of Rif, that it means to collect without an oath. And he notes that this is the usage of the term throughout all of Shas. (Yet here it is the setama rather than in an Amora's original use.) If so, there is a problem -- if all that is being imposed is an oath, even for Abba bar Mar, then why would Rav Pappa not be trusted as a single witness, which would indeed impose a requirement of an oath? The answer Tosafot gives is forced -- that Rav Pappa (and in an analogous situation, Abba bar Mar) was a relative of either the lender or borrower.

The problem with this forced explanation is twofold. Firstly, it is nowhere in the gemara, and this seems like the sort of detail that should be mentioned, either by the setup in the original gemara or in the setama. Secondly and more importantly, Rava does not ask if there is another single witness to stand alone and impose as oath. His words are איכא איניש אחרינא בהדי מר, with the implication that he will work together with Rav Pappa as a second witness. If Rav Pappa were invalid because of being a relative, he could not stand with the other witness. And indeed, as I mentioned above, the intent with that second witness would be to entirely invalidate, and therefore tear up, the shtar.

As such, I would question the setama digmara's emendation here, and say that we should preserve the reading of קרענא.