The ruling on a man having sexual intercourse with his enslaved woman while simultaneously being married to her sister (a free woman).
Chapter on What is Prohibited to Marry and Combining Between Them and Other Matters
Al-Mughni
Book of Marriage
Primary text
If a man has sexual intercourse with his enslaved woman while married to her sister, both women become unlawful for him until he establishes that the enslaved woman is free of pregnancy through Istibra' (waiting period). After Istibra', his free wife becomes lawful for him, but the enslaved woman remains unlawful. This differentiation is because the marriage contract is considered stronger and predates the new illicit act. The Istibra' is obligatory to prevent mixing his semen in the wombs of two sisters.
Supporting text
There is a possibility that both women become unlawful for him until one of them is made lawful, similar to the ruling concerning two enslaved sisters he possesses.