A shomer chinam (an unpaid guard) is exempt from financial responsibility when the guarded object is lost or stolen but is liable if the loss was due to p’shi’ah (negligence). There are two main points of contention to clarify. [We had to omit other, smaller issues in this forum]. One is whether Reuven was a shomer or just a “carrier,” a matter they had not discussed. The second is whether the money’s disappearance resulted from p’shi’ah that occurred after the zipper opened, as Shimon claims, or whether Reuven guarded it in a reasonable manner. Reuven would only have to pay if both points are decided in Shimon’s favor.
Status as a shomer – There is a dispute among Tanna’im[1] regarding one who allows his friend to put an animal in his pen without further stipulation. They disagree whether the owner of the pen accepts responsibility for the animal or simply gives permission without accepting responsibility. The gemara[2] suggests that this is a general machloket whether one who agrees to receive control over another’s property becomes obligated as a shomer even without explicitly accepting that status. It concludes that local, psychological factors may explain the various positions in their specific context.
In any event, the Shulchan Aruch[3] rules that when the wording of the agreement is inexplicit, the holder of the object does not assume the responsibilities of a shomer. However, he continues, if Levi allowed Yehuda to place his shoes on Levi’s donkey before Levi went alone to another city, then, since the shoes would be in a precarious situation if not cared for, we assume that Levi accepted the responsibility of a shomer chinam for the shoes.
Despite similarities to our case, the rationale of the Rosh, the source of this latter halacha, displays differences. Since Shimon accompanied Reuven, Reuven likely intended that Shimon retain responsibility for his items, especially since at the time Shimon put them in the knapsack, it seemed unnecessary for Reuven to give them any further thought. Although the situation became more complex when the zipper broke, the parties’ accounts indicate that Reuven did not intend to accept a new status of shomer as a result.
Was there p’shi’ah? – Reuven is adamant that he was sufficiently careful under the circumstances that arose, whereas Shimon claims that he was not. Ordinarily, a shomer has to make a Torah-level oath that he was not negligent, but since we avoid oaths, there may be grounds for a monetary compromise. However, in this case, neither friend accuses the other of lying; they simply disagree about how to view this borderline case. (As Shimon was aware of Reuven’s actions during the time in question, we can assume that had there been blatant p’shi’ah, Shimon would have taken back his items and/or checked on them earlier.) If there is doubt whether there was p’shi’ah, a shomer is exempt from paying.[4]
Based on the indications (albeit not fully conclusive ones) regarding both issues, and certainly given the convergence of the two, there are not sufficient grounds to require Reuven to pay.