What is the ruling regarding *mahr* when intercourse occurs with a woman whose daughter is prohibited to the perpetrator (e.g., mother-in-law)?

Primary text

According to another narration from Ahmad, if the perpetrator has intercourse with a woman whose daughter is prohibited to him (such as a mother or a daughter), no *mahr* is due. Similarly, if he has intercourse with a woman whose daughter is permissible to him (such as an aunt or maternal aunt), *mahr* is due because the prohibition associated with her is considered lighter.