Gittin 75 - Contracts Based on Unfulfilled Conditions
A man said to his sharecropper, "Sharecroppers usually water the field three times a year and take one-forth of the produce as payment for their work. However, you water four times a year and take one-third of the crop." The sharecropper accepted the proposal. Eventually rain came and made it unnecessary to water the crops for the fourth time.
One side: since the sharecropper did not actually irrigate a fourth time, only one-quarter of the produce is due him. Another: since it was not necessary to irrigate the fourth time, the sharecropper takes one-third. The second point of view is preferred.