Halakhah for Bava Kamma 115:4

The Mishnah, Bava Kamma 115b, describes a situation in which two donkeys belonging to separate masters are in danger of drowning. The Mishnah declares that if the owner of the less valuable donkey offers to rescue the more valuable donkey on the condition that the latter's owner compensate him by the loss of his own less valuable donkey, the rescuer's claim for such compensation is valid. The Gemara, Bava Kamma 116a, reports that Rav Kahana asked Rav whether that is so only if the rescuer's own donkey actually perishes or whether he is entitled to such compensation even if the rescuer's donkey manages to escape drowning on its own accord. The Gemara reports that Rav replied, "Heaven acted favorably toward him," i.e., the contract is enforceable. Or Sameaḥ that the ruling is obviously predicated upon rejection of the claim that the obligation was assumed by the donkey owner only because he believed that the rescuer would suffer a loss as a result of neglecting his own animal but that he had no intention of compensating the rescuer for a loss that did not actually occur.¹³Harei Besamim cites this talmudic discussion but maintains that the rescuer did indeed suffer a loss in abandoning his donkey in turbulent water as a result of which it became res nullius. Reclaiming his donkey afterwards, contends Harei Besamim, is, in fact, tantamount to acquisition of new title to abandoned property. Or Sameaḥ, citing the dialectic of that discussion, argues that the Gemara did not regard the donkey as res nullius. See also the discussion of this issue by R. Moshe Yonah Zweig, Ohel Mosheh, III, no. 33. That claim is impliedly dismissed on the grounds that a fortuitous "act of God" is of no relevance to the express terms of the contract. A fortiori, argues Or Sameaḥ, the lessor's own foresight in seeking insurance is of no relevance to the contract between himself and the lessee.