The Slave is only a slave for a moment in the book. He stays enslaved long enough to fall in love, and for long enough to fall for his temptation to tell Wanda how he feels. Eventually, he is a free man, but he is still bonded to Wanda in his heart. He realizes in a dream that she is pregnant, and she goes to claim her as his wife. Then, like Abraham lied about his marriage in the Bible to protect him and his wife, Jacob does the same, taking Abraham's wife's name as the new name for his own wife: Sarah.
The fact that they lie represents the lack of tolerance in Jacob's community, because as they prove after Sarah's death, they have no tolerance for a Jewish man who would take a wife from the "outsiders." This shows that, although Jacob is a religious man who longs to do the right thing, his community will only accept him if they do exactly what they believe he should.
This dogmatic, closed-minded point of view is given a clever response by the novel when Jacob realizes that his wife is buried among the Jews, even though originally, her bones were outside the cemetery walls. This is because they all died, and the cemetery expanded to include her. In other words, their racism is settled by death itself, a force that doesn't care about racial bias. Everyone dies, so everyone is equal. This is highlighted when Jacob is buried with his wife, like the universe itself is blessing their legacy.