A slave owned by a Jew who is sold to a non-Jew, never mind the penalty in place from the mishnah (on yesterday's daf). The Gemara provides different reasons for why a slave might end up with a new non-Jewish owner - including a Jewish owner trying to pay off debt, or the Sicarii taking the slave as property... Plus, the question of tithing in the context of another taking the property - what happens to the slave who comes alongside? Also, when a slave is sold with the specifics in a document... and the details may not align with the immediate reality, specifically when that means a purchaser who lives outside of the land of Israel.