By the Bais Hora'ah
Many years ago I “borrowed” a Chumash with commentaries from a local beis medrash. I forgot to return it and eventually misplaced the sefer and I do not recall which volume I borrowed.
Q: Am I obligated to purchase the entire set so that I am certain I return the missing sefer?
A: The first issue to discuss is whether borrowing a sefer from a shul without permission is allowed. There are authorities who maintain that there is an ancient takanah (enactment) against removing sefarim from a beis medrash (Gra, C.M. 163:105; Aruch Hashulchan, C.M. 163:17; and Dvar Shmuel 91). Many actually write that there is a cherem (ostracism) in force from previous generations that prohibits removing sefarim from a beis medrash without permission; however, such a cherem has no source or halachic force (Ohel Yehoshua 122).
Nevertheless, the existence of such a cherem is irrelevant since one who borrows something without permission — shoel shelo midaas — is categorized as a thief (C.M. 359:5 [292:1 and 363:5]). Accordingly, regardless of whether the beis medrash is privately owned or community owned, there is no doubt that the owner does not want sefarim removed without permission. Taking a sefer without permission is therefore an act of theft.
Sefarim are purchased for the beis medrash with the intention that they will be available for use there; when a person takes sefarim home he denies others the ability to use them. This is true even when the gabbaim do not protest such forbidden borrowing. A common justification is the principle of neicha leih l’inish l’me’evad mitzvah b’mamonei (people are happy to allow others to use their possessions to fulfill a mitzvah). However, the principle does not apply if the “borrowed” item could become ruined while it is being used to fulfill a mitzvah. Since “borrowed” sefarim often become ruined or lost, the principle is not applicable.
Even the gabbaim may only authorize loans of a sefer for short periods of time, since extended loans prevent others from using it (Ohel Yehoshua, ad loc.).
Someone who “borrowed” a sefer without permission is obligated to return the sefer, the same as any other thief. Even if the owner despaired of retrieving it, e.g., he bought a new set, the obligation to return it remains in force (C.M. 354:2 and 360:5). If the sefer’s condition changed dramatically (shinui) or if it was lost altogether, the “borrower” is obligated to repay the value of the sefer (C.M. 360:5).
In your case, since the sefer you borrowed is missing, there is no obligation to replace the actual sefer. Your obligation is to repay the beis medrash the sefer’s value at the time you “borrowed” it. In other words, you would pay the value of a used sefer rather than a new sefer, and there is no need to pay for a complete set.