פירוש על עבודה זרה 11:15

If the idolater has another animal of his own, then if the prohibition is due to profit, it is still in effect because by selling the idolater another animal, the Jew still causes him to profit. If, however, the prohibition is because of placing a stumbling block before the blind, then if the idolater has another animal to sacrifice, the Jew is not enabling him to offer a sacrifice. The idolater could have sacrificed one anyway.