שֶׁלֹּא יִמְכֹּר אָמָה עִבְרִיָּה הַקּוֹנֶה אוֹתָהּ מִיַּד הָאָב – שֶׁכָּל מִי שֶׁיִּקְנֶה אָמָה עִבְרִיָּה לֹא יִמְכְּרֶנָּה לְאָדָם אַחֵר לְעוֹלָם, שֶׁנֶּאֱמַר (שמות כא ח) לְעַם נָכְרִי לֹא יִמְשֹׁל לְמָכְרָהּ וְגוֹ', וּפֵרוּשׁוֹ כְּתַרְגּוּמוֹ, לִגְבַר אָחֳרָן. וּלְהַרְחִיק הַדָּבָר נֶאֱמַר בְּלָשׁוֹן זֶה, כְּלוֹמַר שֶׁדּוֹמָה לַעֲנִיָּה הַקְּטַנָּה אִם מוֹכְרָהּ לָאָדָם אַחֵר שֵׁנִית כְּאִלּוּ יִמְכְּרֶנָּה לְעַם נָכְרִי. That the one who acquires the Hebrew bondwoman from the father not sell her: That anyone who acquires a Hebrew bondwoman not sell her to another man ever, as it is stated (Exodus 21:8), “he shall not have the right to sell her to a foreign people, etc.” And the explanation [of the phrase] is like [Onkelos’ Aramaic] translation, “to another man.” And it was stated with this wording to distance this thing — meaning to say that if he sells her to a second man, it is for this poor little one similar to if he sold her to a foreign people.
מִשָּׁרְשֵׁי הַמִּצְוָה, שֶׁרָצָה הָאֵל לְזַכּוֹתֵנוּ וְצִוָּנוּ לְהִתְנַהֵג בְּמִדַּת הַחֶמְלָה הָאֲהוּבָה לְפָנָיו. From the roots of this commandment is that God wanted to give us merit and commanded us to act with the trait of compassion, which is beloved in front of Him.
דִּינֵי הַמִּצְוָה כָּתוּב לְמַעְלָה (מצוה מג) בְּמִצְוַת יִעוּד (ה' עבדים פ"ד). The laws of the commandment are written above in the commandment of designation (Sefer HaChinukh 43). (See Mishneh Torah, Laws of Slaves 4.)