Rif: The Din In The Gemara is One Way, But We Rule In Another Way

We are soon to encounter a noteworthy Rif. He argues with other post-Talmudic Rabbis about the meaning of a gemara, such that the din in the gemara would be other than what they say. They say that decreases in value of nichsei tzon barzel, iron sheep brought into marriage, are not the loss of the husband. Rif argues that according to the correct interpretation of the gemara, such losses are borne by the husband. Yet, because the halacha has been misunderstood otherwise until this point, this misunderstanding forms the basis of the husband's understanding when entering into marriage and establishing the ketuba. If so, the assumptions of the husband are the basis for how we rule in this monetary matter.

This approach can have wide ranging impact on dinei mamonot, monetary matters. Indeed, while I have not learned them in depth, this may well contribute to a general import laid on precedent in דיני ממונות.

At any rate, this Rif is here. It will show up in a few days on my Alfasi blog, on 21b-22a in the pages of Rif, about approximately Yevamot 66b:

ודייקי רבוואתא מדקתני במתניתין בעבדי צאן ברזל אם מתו מתו לו ולא קתני אם פחתו פחתו לו דוקא כעין מתו ממש דלא
משמשי מעין מלאכתן כלל דכמאן דליתנהו דאמי הוא דחייב באחריותן ומשלם דמים אבל אי איתנהו אף על גב דבלו טובא ופחתו אי משמשי מעין מלאכתן נוטלתן האשה ואינו משלם דמים שדין עבדי צאן ברזל ודין נכסי צאן ברזל אחד הוא כדין אלו כך דין אלו

And the {post-Talmudic} Rabbis deduced from the fact that it said in our Mishna regarding servants who are iron sheep, "if they died, they died to him," and it does not teach "if they reduced in value, they reduced in value to him," that specifically cases similar to their actual death, such that they cannot do anything like their work at all, and so they are like they are not there at all -- in such a case he is liable in their responsibility and pays money, but if they are there, even though they greatly degraded and reduced in value, if they can serve in a way similar to their work, the woman takes them and he {the husband} does not pay money. For the law of servants who are iron sheep, and the law of assets which are iron sheep is one and the same. As the law of these, so is the law of these.

ואנן לא חזינן להאי דיוקא

דהא מקשינן בגמרא למאן דאמר כל היכא דמיחייב באחריותן אכלי בתרומה מהא דתנן

כהן ששכר פרה מישראל אע"פ שמזונותיה עליו לא יאכילנה בכרשיני תרומה
וקא מפרקינן ותיסברא נהי דמחייב באונסיה בכחישה וביתרות דמיה מי מחייב
מדקא מפרקי' הכי גבי שכירות פרה מכלל דלגבי צאן ברזל מיחייב אפילו בכחישה וביתרות דמים ולהכי אכלי בתרומה
ושמעת מינה דליתא להאי דיוקא דדקו קמאי ז"ל
אלא מיהו אע"ג דדינא דגמרא הכי לא מחייבינן ליה לבעל השתא בכחישה וביתרות דמים דכיון דלא נהגי עלמא הכי כל צאן דמקבל לנכסי צאן ברזל וכתב להו עליה אדעתא דמנהגא הוא דמקבל להו עליה הילכך לא מיחייב אלא לפום מנהגא

And we do not see {agree with} this deduction. For we ask in the gemara on the one who said that where he is obligated in their responsibility, they may eat of teruma, from this that they learn {in a Mishna}: A kohen who rented a cow from an Israelite, even though he is obligated in feeding it, he may not feed it vetches of teruma. And we resolve it as follows: Think now! Although he is liable {our gemara: for its theft or loss; I would suggest: for its feed}, but for its accident, emaciation or its reduction in value do we obligate? {No!}
From the fact that we resolve it like that by renting a cow, we may deduce that by iron sheep, he is obligated even for emaciation and reduction in value, and that is why they eat teruma.
And we deduce from this that there is nothing to this inference that they, of blessed memory, deduced before us.
However, even though the law in the gemara is such, we do not require the husband nowadays in emaciation and reduction of value, for since the world does not practice in accordance to this, all sheep that he accepts as iron sheep and writes them down for her, it is in mind of this custom {/practice} that he accepts he accepts upon himself to her. Therefore, he is only obligated in accordance with the custom.