פירוש על עבודה זרה 14:9

There is a prohibition of selling large animals to idolaters. The issue seems to be that the idolater will use the animal on Shabbat. We will discuss this more later in the tractate. But Nahum the Mede allows one to sell an old horse during times of war, for such a horse will not have much use and may even end up killing its owner. People won’t learn from this rare case that it is permitted to sell large animals in general.