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Rav Nachman says: A person can ask his friend to borrow something “b’tuvo” — “when good,” and (if his friend acquiesces) borrow it anytime thereafter (even after he returns it, provided that it is in good condition, for that is what he meant when he said “b’tuvo” ).
Rav Mari, the son of the daughter of Shmuel, said: This is only if they made a kinyan to this effect.
The Ritva writes that the main novelty of this halachah, according to Rashi’s explanation, is that we expound his language. We assume that this was the borrower’s intent when he used the word “b’tuvo.”
Tosfos cites an Aruch, who offers a different explanation. If the borrower did a favor for the lender, the borrower may use the borrowed utensil forever.
The Ritva explains: The borrower, after doing a favor to the lender, requests of him to borrow his utensil in exchange for the favor that he did. The novelty of this halachah is that he is not regarded as a renter, even though the utensil is being lent to him in exchange of the favor which he provided. He is, nevertheless, regarded as a borrower.
The Rif suggests an alternative explanation of the Gemora: The borrower said to the lender, “Lend me, on account of your goodness, this utensil that I can use it.” Since a term limit was not mentioned, he may borrow it forever, or until it breaks. The novelty of this ruling is that by adding the words, “your goodness,” he may borrow it forever. This is unlike an ordinary unspecified borrowing, which is only for thirty days.
The Rosh asks: Why do these words make a difference? It is extremely common for someone to say to his fellow, “Can you do me a favor please and lend me a utensil?”
The Sm”a answers that we are referring to a case where the borrower said to the lender, “You are such a good person, one who is not particular at all as to the amount of time your utensil is lent out.” In this case, we assume that he is lending it forever.