Siman 318

One who rents a mill from his fellow on the condition that he grinds twenty se'ah for him every month as his rent for what he needs for his household, and the owner of the mill became wealthy and behold he does not need to grind there because he eats grain that is better than what is ground in his own mill—if the renter has wheat that he needs to grind for himself or for others in place of what he would have ground for the lessor, we compel him to give the lessor the value of grinding twenty se'ah. For since he suffers no loss in this, it is the "trait of Sodom" if he does not agree to this, and we compel regarding the trait of Sodom. But if he does not have others to grind in their place, the renter may say: I will not give you money; rather, behold I will grind for you as I contracted, and if you do not need it for yourself, go and sell it to others.

And if the lessor cannot find someone to sell to, there are some of our teachers who hold that even though the lessor cannot compel the lessee to suffer a loss through him, nevertheless, he also has the power to cancel the rental and take the mill from his hand within its term, for he too should not have to suffer a loss [Tosafot Ketubot 103a]. And there are some of our teachers who do not hold so, and hold that he does not have the power to cancel the rental, for since the lessee did not change anything, but rather the lessor's situation changed, by what right is his power sufficient to cancel the rental within its term? [The Rosh ibid.]. And in truth, the first opinion is puzzling, for even if the house of a lessor fell, he cannot remove the lessee from his house, as I wrote at the beginning of siman 312. And it is a forced explanation to say that they hold like the approach of the Yerushalmi there, that in a case of total loss one cancels the rental. Therefore, some say that they do not disagree at all; for if it is the way of the wealthy not to eat from such grinding, and only the poor eat them, and accordingly, when he mentioned to him "on condition to grind for him for his household needs," it is as if he said "as long as I need it for my household needs" [it appears to me]. Therefore, when he became wealthy and is unable to eat them, it is like one who sold and did not need the money, where the sale is returned, as I wrote at the beginning of siman 207, and the rental is canceled. But if the wealthy also eat from such grinding, but this lessor in his wealth desires to obtain it from another place, he does not have the power to cancel the rental even though he will suffer a loss thereby [Taz]. And see what I wrote in siman 60 regarding one who obligated himself to give his fellow sustenance and requests that he give him the value of the sustenance.