Responsa על בבא מציעא 72:20

Q. A demanded from B the book of the order Kodashim which he lent him to copy from. B, in turn, demanded from A the book of the order Moed which he gave to A as security, and which was worth twice as much as A's book. At first A denied that he received B's book, but when confronted by a witness who supported B's claim, A retracted his denial, but alleged: 1) that B loaned him the book with the understanding that his (A's) children should be permitted to study therefrom; 2) that he loaned the book to his son-in-law; and 3) that the book was subsequently burned in the latter's house, and that hence the loss of the book was an unavoidable accident. B denied that he gave A permission to allow his sons to use the book.
A. Since A and B lent books to each other, their responsibilities regarding these books were those of hired watchmen who are not responsible for unavoidable accidents. The question revolves, then, about A's lending the book to his son-in-law. Therefore, upon A's taking an oath that B gave him permission to allow his sons to use B's book, A will be free from any obligation to B. B, on the other hand, may retain A's book, upon taking an oath that he did not permit A to give his book to A's sons for study, and B is further entitled to collect from A's son-in-law (the difference between the value of the lost book and the one B retained) since the responsibility of the son-in-law regarding the book was that of a borrower who is responsible even for unavoidable accidents. If, however, A's son-in-law is willing to take an oath that he did not borrow B's book from A, he will be free from obligation to B, and his oath will not subject his father-in-law to any further obligation.
The question was submitted by R. Moses Azriel b. R. Eliezer Darshan, whose opinion, appended to the question, coincided with that of R. Meir.
SOURCES: Pr. 963; Mord. B. M. 282. Cf. Isserlein Pesakim 200.

Q. When A was imprisoned the community delegated B to endeavor to release him. A claims that he sent books to B, which were valued at twelve marks, so that B, using the books as security, might procure a loan from the charity-chest at its regular rate of interest; and that he, thus, owes B nine pounds Nürnburg coins, which he is ready to repay. He demands that B permit him to redeem his books. B claims: a) that when A gave him the books, he, A, undertook to make good all damages incurred in securing the loan; b) that he, B, suffered damages because of his endeavors on A's behalf; c) that A sent word to R. Yedidyah and to B, to sell two books; d) that he, B, gave the two books to Rabbi Yedidyah who sold them, but who now refuses to give the money to B. A, however, claims that he only advised B to secure the aid of Rabbi Yedidyah in selling the books, since R. Yedidyah knew their value and how to sell them, but that he did not permit B to give the books to R. Yedidyah. Moreover, A claims that R. Yedidyah retains the money he received for the two books, for a debt B owes him. B denies that he owes money to R. Yedidyah.
A. If B admits that he received A's books, he is responsible for them. If, however, B claims that A gave him permission to give the books to R. Yedidyah, he must take an oath to that effect and then be free from obligation. Moreover, if A usually entrusted his valuables to R. Yedidyah, B is free from obligation without taking an oath. As to the damages B claims to have suffered on A's accounts because he went surety for A to Gentiles, if he has in his possession money belonging to A, he may take an oath as to the amount of damages he suffered, and be entitled to retain that sum. For if one Jew goes surety to a Gentile for another Jew, the latter must make good all damages suffered by the surety because of his suretyship, excepting damages caused by his wilful neglect. At first I was told that this law was an ordinance enacted by the communities, but now I realize that it is sound talmudic law (based on B. K. 114a: "thou hast placed a lion next to my field"). If, however, B has in his possession no money belonging to A, A may take an oath that he expressly undertook to be responsible only for the interest of a loan secured from the charity-chest, and be obligated to pay B only such interest.
SOURCES: Pr. 977.