פירוש על בבא מציעא 8:8

נתעברה לשוכר – he would not increase the monthly payment, for the intercalation is included in the year.

If one rented a house to his fellow by the year and the year was made a leap year, the extra month goes to the tenant. If he rented it by the month and the year was made a leap year, the extra month goes to the owner. It once happened in Tzippori that a person leased a bath-house from his fellow at “twelve golden dinars a year, one dinar per month”, and [when the year became a leap year] the case came before Rabban Shimon ben Gamaliel and Rabbi Yose, and they said: “Let them share the extra month.” In the time of the mishnah there was no fixed calendar as we have in our day. Rather each year they would decide whether to make a leap year of 13 months or not. The Hebrew months are lunar months, of either 28 or 29 days, making a total of 354 days per year. Since this is 11 days short or a regular solar year, the Hebrew year must receive an additional month about once every three years. If one leased a house by a yearly wage and the year became a leap year, the tenant benefits by receiving an extra month for free. Since the terms of the contract were for a year, the tenant receives a year, whether it is a leap year or not. If, however, he leased the house by the month, and the year became a leap year, he must pay for the extra month. In the story (1c), the rental agreement for the bath-house contained ambiguous language, stating both the yearly rate and the monthly rate. Since it was unclear whether the owner had rented the house on a yearly or monthly rate, they split the cost of the extra month.

מעשה בציפורי – In the Gemara (Bava Metzia 102b), it raises an objection. You quote a story which disproves your rule! For the first part of the Mishnah teaches either it all goes to the renter or all goes to the person who rents it out, and it brings a story/case where they should divide it, and it responds that the Mishnah is deficient and it should be read as follows: If he said to him: “I rent it to you] for twelve golden denarii per year, at a golden dinar per month, they must divide/share it between them. And we don’t know if he seized the first language or the latter language, and the story also, etc. But the Halakha is not according the Rabban Shimon ben Gamaliel or like Rabbi Yosi, but one goes after the least of these languages, that the land is in the possession of its owner stands. Therefore, all of it is [belongs] to the person renting out, whether the first language was less whether the latter language was less.