Responsa על בבא מציעא 112:18

Your reasoning and the fine distinctions you draw are correct, but they apply only to a case where the person who is seeking to sell the house that he has rented to another, is not forced to sell the house by dire necessity. When a person is thus forced to sell his house, we assume that originally when he rented the house to his tenant he did not intend to let it remain rented while he was dying from hunger. Therefore, if the landlord take an oath to the effect that he is forced by dire necessity to sell his house, his tenant will have to vacate the house. If the tenant has paid his rent in advance for a certain period, however, he may not be forced to vacate before the expiration of that term, since renting is a form of buying limited to the period for which the rent is paid. In our case, the tenant may not claim that his landlord is not in desperate need of money since he owns real estate on which his wife's ketubah is a lien, and he could sell to another the right to take title to such real estate in case his wife die before him, for we assume that at the time the landlord rented his house to the tenant he did not intend that the renting should cause him to jeopardize his property in such a manner.
This Responsum is addressed "to my teacher Rabbi Asher."
SOURCES: Am II, 241; cf. Mord. B. M. 383; Asheri B. M. 25.

Q. A claims that B had promised to pay him half a mark for accompanying him on his ride to a certain place. He rode with B to that place, and now demands his pay. B avers that the offer had been made in jest.
A. Since A acted upon B's offer, the latter cannot, now, dismiss such offer as a mere jest. However, A is not entitled to receive the full amount offered, if such amount exceeds the actual value of his services by more than one-sixth. If such is the case, A is entitled to the actual value of his services only.
This Responsum is addressed to "my relative R. Kalonymus."
SOURCES: B. p. 294 no. 372.