Is the marriage to a sister permissible if the husband divorces or sells his previously owned enslaved sister?
Chapter on What is Prohibited to Marry and Combining Between Them and Other Matters
Al-Mughni
Book of Marriage
Primary text
If the husband of an enslaved woman he has had relations with divorces her or transfers ownership of her, he is permitted to marry her sister. If the enslaved woman returns to his ownership, the marriage remains valid, and her freedom is preserved because the marriage contract is sounder and stronger. The enslaved woman remains unlawful for him [to marry if he had relations with her sister first, as implied by subsequent contexts, but the immediate ruling here is about the initial sister]. The evidence is that the marriage contract is stronger, and her status remains as it was.
Supporting text
There is an alternative view suggesting that one of the two sisters should become unlawful for him because the enslaved woman with whom he had relations has returned to his possession, and the one he married is still his wife (or in a similar state of intimacy), resembling the situation where he owned two enslaved women, had relations with one after marrying the other, and then the husband divorced the sister (the second one married).