Commentary for Kiddushin 23:1
לא ס"ד דומיא דייעוד מה ייעוד אע"ג דאי בעי מייעד ואי בעי לא מייעד כל היכא דלא מצי מייעד לא הוו זבינא זביני ה"נ כל היכא דלא מצי מיגרעא לא הוו זבינא זביני
- You cannot think so, [for] it is similar to designation:<span class="x" onmousemove="('comment',' sugh or sughh, the betrothal of a Hebrew handmaid to her master or his son in virtue of having been bought, no other than the purchase money being necessary.');"><sup>1</sup></span> just as designation, though he [the master] can designate her or not, as he will, yet where he may not designate her, the sale is invalid;<span class="x" onmousemove="('comment',' I.e., she cannot be sold, e.g., to her brother, since she may not be designated to him.');"><sup>2</sup></span> so here too, where he cannot deduct, the sale is invalid.<span class="x" onmousemove="('comment',' Since Scripture teaches that a deduction is made, the sale must be capable of one.');"><sup>3</sup></span>
Daf Shevui to Kiddushin
I should reemphasize—by rabbinic times such institutions such as selling daughters into slavery did not exist. They are there in the Torah, and therefore rabbis interpret them, but they did not practice them in their real lives.