(Mishnah): If a flooding river overcame the donkeys of Reuven and Shimon, and Reuven neglected his own donkey (worth 100) to save Shimon's (worth 200), he is paid only like a worker (he is not compensated for the loss of his donkey).

If he said 'I will save yours and you will pay for mine', Shimon must pay him.

(Gemara - Beraisa): If Reuven neglected his own in order to save Shimon's, and Shimon's came out [of the water] by itself, he need not pay [Reuven] anything.

Question: If Reuven [said 'I will save yours and you will pay for mine', and he] neglected his own donkey in order to save Shimon's, and Reuven's came out by itself, can [Reuven] tell him 'I despaired from my donkey' (so you must pay like you agreed; even though mine came out, Hash-m was gracious and gave a gift to me)?

Answer: We learn from the following. If a flooding river overcame one's donkey, and he screamed 'I do not despair!', Reish Lakish says, as long as he screams, he did not despair;

R. Yochanan says, the Chazakah is that he despaired. (According to R. Yochanan, also here we say that Reuven despaired, and Shimon must pay him for his donkey.)