Commentary for Avodah Zarah 40:16

When the Jew sells a tree, grain, or low-growth to the non-Jew while still attached to the land, the Jew still owns the land. He will have some leverage over the non-Jew to get him to cut it down. This might be why R. Judah allows one to sell the vegetation to the non-Jew. But if he sells an animal on condition that it be slaughtered, he will have little leverage. The non-Jew might hold on to it for a long period of time, and perhaps work the animal on Shabbat. In this case, perhaps R. Judah would hold that it can’t be sold. Or maybe it makes no difference, and R. Judah would permit here as well.