بسم الله الرحمن الرحيم
(Fatwa: 931/860/SN=8/1438)
In Hadith and biography books, we did not find the mention of mahr or dowry in accounts of Hadhrat Ruqayya (may Allah be pleased with her) and her nikah. However, according to one narration narrated by Hadhrat Umar Ibn al-Khattab, may Allah be pleased with him, says that the mahr of all the wives and daughters of the Holy Prophet Muhammad (may Allah be pleased with them) except Hadhrat Umm Habibah, may Allah be pleased with her was not more than 12 Uqia (1.5 kg, 30 gram and 90 milligram silver). It means that the mahr of Hadhrat Ruqayyah (may Allah be pleased with her) might have been the same.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband