Does sexual intercourse with one of two related women (e.g., sisters) prohibit intercourse with the other before the first one becomes permanently unlawful for him?
Chapter on What is Prohibited to Marry and Combining Between Them and Other Matters
Al-Mughni
Book of Marriage
Primary text
Intercourse with one of two related women does not permit intercourse with the other until the first woman is permanently removed from his ownership, such as through manumission or marriage. This is the position held by Ali, Ibn Umar, Al-Hasan, Al-Awza'i, Ishaq, and Al-Shafi'i. If he mortgages one woman, her sister does not become lawful for him because the prohibition stems from the right of the pledgee (mortgagee), not from a permanent prohibition on her person. This is evidenced by the fact that if the pledgee permits intercourse, it becomes lawful, and because he retains the ability to redeem her at any time. There is no specific Quranic or Hadith evidence mentioned to support this main opinion beyond attributing it to the cited scholars.
Supporting text
Qatadah stated that if he performs Istibra' (seeking purification after sexual contact), the other woman becomes lawful for him, arguing that his proprietary relationship is dissolved. This is likened to the situation where a child born from such contact is legally negated through Istibra'. An opposing view suggests that if he imposes a vow that prohibits one woman, the other does not become lawful, because a mere oath is revocable through expiation (kaffarah), akin to menstruation, ihram, or fasting.