The phrase "and he shall go out alone" in (Exodus 21:4) seems redundant. If the bondsman's term is up, of course he goes out. Why add "alone"? The Mekhilta finds hidden legal content in this apparent surplus.
The Torah establishes in (Exodus 21:2) that a Hebrew bondsman serves six years. During this time, the master may provide him with a Canaanite bondswoman. But what about a bondsman whose ear has been bored — one who chose to extend his service beyond six years? Can his master also give this type of bondsman a Canaanite bondswoman?
The verse "six years shall he serve" might suggest that only the standard six-year bondsman is eligible for this arrangement. The bored-ear bondsman operates under a different framework — he stays until the Jubilee year. One might conclude that the master's right to provide a bondswoman applies only to the first category.
The phrase "and he shall go out alone" corrects this. By specifying that the bondsman goes out "alone" — meaning without the bondswoman and her children — the Torah implies that the bondswoman provision applies to him too. You only need to say someone goes out "alone" if there were others he might have gone out with. This includes the bored-ear bondsman. His master can also give him a Canaanite bondswoman during his extended service, and when he eventually departs, he leaves her and her children behind.