"And if a man sells his daughter" (Exodus 21:7) — the Torah permits a father to sell his daughter as a maidservant. The Mekhilta immediately asks: can a mother do the same? The answer is no, and the proof requires a logical argument.
One might reason as follows: a man is not permitted to sell his son, yet he is permitted to sell his daughter. If a man, who lacks the power to sell sons, can sell daughters, then surely a woman — who might be expected to have comparable authority over her children — should also be able to sell her daughter.
The Torah preempts this logic by specifying: "And if a man sells his daughter." The word "man" is deliberate and exclusive. A man may sell his daughter. A woman may not. The verse does not say "if a parent sells" or "if one sells." It says "a man."
This restriction limited the circumstances under which a child could be sold into servitude. Only the father held this power, and even then, Jewish law surrounded the sale with extensive protections for the daughter — including the possibility of betrothal to the buyer or his son, and automatic release at puberty. By restricting the power of sale to the father alone, the Torah reduced the number of people who could initiate this transaction and ensured that at least one layer of protection — paternal judgment — stood between the daughter and servitude.