(Exodus 21:3) introduces a condition for the Hebrew bondsman: "If alone he came, alone shall he go out." The Mekhilta uses this verse to determine whether a master is required or merely permitted to give his bondsman a Canaanite maidservant during his period of service.
The Torah later states in (Exodus 21:4): "If his master gives him a woman" — referring to a Canaanite bondswoman. The word "if" here suggests optionality. The master may give such a woman but is not required to. Rabbi Yishmael taught: you say this is optional, but perhaps it is mandatory?
The proof comes from "if alone he came, alone shall he go out." This verse establishes the default: if the bondsman entered service unmarried, he leaves unmarried. The fact that Scripture envisions the bondsman departing "alone" — in the same state he arrived — confirms that giving him a bondswoman was never a requirement. It was a choice the master could make.
This matters because any children born from the union between the bondsman and the Canaanite bondswoman would belong to the master. A mandatory pairing would effectively force the bondsman into producing children who would remain enslaved even after his own release. By establishing that the arrangement was optional, Rabbi Yishmael's reading preserved the bondsman's autonomy. No one could compel him into a relationship whose children he would have to leave behind.