פירוש על עבודה זרה 30:2

Rabbah points out that the cases are not the same. One is not commanded to make sure that one’s cattle rests on the Sabbatical year. It is a prohibition that falls on the person, not on the cattle. But when it comes to Shabbat, one is commanded not to let one’s cattle work on Shabbat. Therefore, Rabbah argues, a Jew cannot sell his animal to an idolater.