The Mishnah teaches on 115b that if a person saves his friend's donkey or wine and abandons his own donkey or wine in order to do so, if he does not stipulate that the friend should reimburse him, "Ein Lo Ela Secharo." If he does stipulate that "you will give me mine in exchange", the friend must pay him.
Why can't he stipulate that the friend will not only reimburse him for his lost donkey or wine, but also pay him the Sechar Tircha for involving himself in saving other person's property (Secharo of the Reisha)?
Reuven Winer
If the Halachah is as you say, it is because it is a Mitzvah of "Hashavas Aveidah" once he does not sustain a loss by saving the other person's property, and therefore there is no Chiyuv to pay for the Tircha (and one should not demand compensation for it, l'Chatchilah) once the loss is reimbursed.
(Of course, in such a case if he made a Tenai, the owner will be Chayav and not be able to say "Meshateh Ani Bach"). However, in the Reisha, since there the one saving his friend's property is himself sustaining a loss, he is not Mechuyav in "Hashavas Aveidah."
D. Zupnik