The Torah uses the phrase "who did not designate her" in reference to a Hebrew maid-servant whose master has not taken her as his wife (Exodus 21:8). The Mekhilta unpacks this phrase through multiple interpretations, each revealing a different legal principle about the protections afforded to a young woman sold into servitude.
The first reading is straightforward. If the master does not designate her as his wife, meaning he does not take her in marriage as the Torah anticipated, then her father should redeem her. The master's failure to fulfill the implicit purpose of the sale triggers the father's obligation to buy her back. She should not remain in a household where the original intention has been abandoned.
The second reading draws a different rule from the same words. "Who did not designate her" teaches that a master should not designate two maid-servants as wives simultaneously. The singular "her" implies exclusivity. If he chooses to exercise his right of marriage, it must be directed at one person at a time.
Rabbi Yossi adds another dimension. A father should not sell his daughter on the explicit condition that the master take her as a wife. The sale and the marriage must remain legally separate transactions. A father sells his daughter into service. If the master later chooses to marry her, that is a distinct act. Bundling the two together from the outset is forbidden.
Rabbi Akiva disagrees. He rules that the father may sell his daughter on this condition. If the master wishes to fulfill it, he marries her. If he does not, the sale is annulled entirely. Rabbi Akiva treats the conditional sale as valid, with the condition functioning as a built-in exit clause that protects the daughter if the expected marriage never materializes.