Bava Metzia 79B

נימא ליה: הב לי ההוא חמרא ואנא מייתינא ספינתא.

Let the owner say to him: Give me that wine and I will bring a boat. Since that particular wine is gone, the renter cannot comply with his request, and therefore he should have to pay the rental fee.

אמר רב פפא: לא משכחת לה אלא בספינה זו ויין זה. אבל בספינה סתם ויין סתם ⁠– חולקין.

Rav Pappa said: You find the correct application of R' Natan’s ruling only in a case where the two parties stipulated this specific boat and that specific wine. Since neither party can fulfill his part of the agreement, the money remains where it is. But if they stipulated an unspecified boat and unspecified wine, as they can both complete the agreement, they divide the rental fee, i.e., the renter pays half.

תנו רבנן: השוכר את הספינה ופרקה לה בחצי הדרך ⁠– נותן לו שכרו של חצי הדרך, ואין לו עליו אלא תרעומת. היכי דמי? אילימא דקא משכח לאגורה ⁠– אמאי אית ליה תרעומת? ואי דלא קא משכח לאגורה ⁠– כוליה אגרה בעי שלומי!

A baraita states: With regard to one who rents a boat and unloads it [uferakah] halfway through the journey, the renter gives the owner his rental fee for half the journey, and the owner of the boat has nothing but a grievance against him. The Talmud asks: What are the circumstances? If we say that this is referring to a situation where the owner can find someone else to whom he can rent out the boat, why does he have grounds for a grievance? And if it is referring to a case where he cannot find another to whom he can rent it out, the renter should be required to pay the full rental fee, as he reneged on his commitment to rent the boat for the entire trip.

לעולם דקא משכח לאגורה, אלא אמאי אית ליה תרעומת ⁠– משום רפסתא דספינתא. אי הכי, טענתא מעלייתא הוא וממונא אית ליה גביה.

The Talmud answers: Actually, this is referring to a case where the owner can find someone else to whom he can rent out the boat. But why does he have cause for a grievance? Because of the wear and tear on the boat due to the additional loading and unloading of cargo, which was not taken into account in their agreement. The Talmud asks: If so, that is a proper legal claim, and the owner of the boat has not merely a grievance against the renter, but cause for claiming monetary restitution from him.