Rif Ketubot 6a {Ketubot 15b; 18b - 19a}

6a

{Ketubot 15b}
Mishna:

ומודה ר' יהושע באומר לחברו שדה זו של אביך היתה ולקחתיה ממנו נאמן שהפה שאסר הוא הפה שהתיר
ואם יש עדים שהיתה של אביו והוא אומר לקחתיה ממנו אינו נאמן:

And Rabbi Yehoshua admits, where he said to his friend "This field was your father's and I acquired it from him" that he is believed {Ketubot 16a} for the mouth which bound was the mouth that loosened.
And if there are witnesses that it was his father's, and he says "I acquired it from him," he is not believed.

{Ketubot 18b}

העדים שאמרו כתב ידינו הוא זה אבל אנוסין היינו קטנים היינו פסולי עדות היינו הרי אלו נאמנין ואם יש עדים שהוא כתב ידן או שהיה כתב ידן יוצא ממקום אחר אינן נאמנין

Witnesses who said "this is our handwriting, but we were forced; we were minors; we were invalid witnesses," they are believed. But if there are witnesses that it is their handwriting, or there other handwriting {sample} comes from another place, they are not believed.

Gemara:

אמר רמי בר חמא לא שנו אלא שאמרו אנוסין היינו מחמת נפשות אבל אנוסין היינו מחמת ממון אינן נאמנין מאי טעמא לפי שאין אדם משים עצמו רשע:

Rami bar Chama said: The only learned this where they said "we were forced by mortal danger," but "we were forced via threat of money," they are not believed.
What is the reason? Because a man cannot declare himself an evildoer.

{Ketubot 19a}

אמר רב יהודה אמר רב האומר שטר אמנה הוא זה אינו נאמן
דקאמר מאן אילימא דקאמר לוה פשיטא לאו כל כמיניה
ואלא דקאמר מלוה תבא עליו ברכה
ואלא דקאמרי עדים אי דכתב ידן יוצא ממקום אחר פשיטא
ואי אין כתב ידן יוצא ממקום אחר אמאי לא מהימני
אמר אביי לעולם דקאמר מלוה וכגון שחייב לאחרים וכדר' נתן
דא"ר נתן מניין לנושה בחבירו מנה וחבירו בחבירו שמוציאין מזה ונותנין לזה שנאמר ונתן לאשר אשם לו

Rav Yehuda cited Rav: If one says "this is a {loan} deed of trust" {where he writes this document trusting that the loan will be given to him later}, he is not believed.
In this case, who is speaking? If it is the borrower speaking, it is obvious that he is not believed! But rather, if it is the lender, may blessings come upon him {for being so honest, such that he will now not collect}. But rather witnesses? If their handwriting comes from another place {such that we need not rely upon them for that fact}, it is obvious! And if their handwriting does not come from another place, why should they not be believed?

Abaye said: In reality, it is where the lender said this, and such that he is obligated to others, like this of R' Natan. For R' Natan said: From where {do we know} that if one {A} has a claim of 100 against his friend {B}, and his friend {B, has such a claim} against his friend {C}, that we can take from this one {C} and give to that one {A}? For it is written {Bemidbar 5:7}:

ז וְהִתְוַדּוּ, אֶת-חַטָּאתָם אֲשֶׁר עָשׂוּ, וְהֵשִׁיב אֶת-אֲשָׁמוֹ בְּרֹאשׁוֹ, וַחֲמִישִׁתוֹ יֹסֵף עָלָיו; וְנָתַן, לַאֲשֶׁר אָשַׁם לוֹ.

7 then they shall confess their sin which they have done; and he shall make restitution for his guilt in full, and add unto it the fifth part thereof, and give it unto him in respect of whom he hath been guilty.