Midras Keluaran 21:7: Mekhilta DeRabbi Yishmael, Midrash Tanchuma Buber & Shemot Rabbah

For it is written (Ibid. 7) "And if a man sells his daughter as a maidservant, she shall not go out as the (Canaanite) bondsmen go out" — by (loss of) organ prominences, as the Canaanite bondsmen do go out. You say (it means) this; but perhaps (it means that) she should not go out with (six) years (of service) or with Jubilee years, as the (Hebrew) bondsmen do go out. It is, therefore, written (Devarim 15:12) "If your Hebrew brother or sister is sold to you, etc." — whereby we are apprised that she does go out with (six) years (of service). Whence do we derive (the same for) the Jubilee? From (Leviticus 25:42) "For they (i.e., all Hebrews are My servants." In any event, (it has been shown that) it is not the second conclusion (i.e., that she does not go out with years of service and with Jubilee), which is correct, but the first (i.e., that she does not go out with loss of organ prominences).

(Exodus 21:7) "And if a man sells his daughter": Scripture speaks of a minor (under twelve). You say that it speaks of a minor, but perhaps it speaks of an adult! — Would you say that? (The rationale for his selling is) Since he is permitted to annul her vows, he is permitted to sell her. Just as he may annul the vows of a minor but not of an adult, so, he can sell a minor but not an adult. But (reason from) the "place" you are coming from (i.e., from vows). Just as there, (he may annul her vows) when she is a na'arah, (a maiden), (from twelve years and a day until twelve and a half years, viz. [Numbers 30:17]) here, too, (he should be able to sell her) when she is a na'arah! — Would you say that? if (pubertal) signs remove her from servitude, how much more so (may he not sell her [a na'arah]) when she has not yet been sold!

(Exodus 21:7) "And if a man sells his daughter as a maid-servant" — He sells her once as a maid-servant, and not twice. R. Yossi Haglili says: We find, then, that he (a father) can betroth her betrothal after betrothal (i.e., if she were divorced after the first betrothal, he may betroth her to another), and betrothal after (having sold her as a) maid-servant, but not maid-servant after maid-servant, and, it goes without saying, not maid-servant after betrothal.