The Emigration and the Apostasy
Umm Habiba emigrated to Abyssinia with her husband Ubayd Allah ibn Jahsh as part of the early Muslim migration to the Christian kingdom of the Negus. In Abyssinia, her husband converted to Christianity — abandoning Islam.
This left her in an impossible situation: her husband had apostatized; they were in a foreign kingdom; she had no way to return to Mecca where her father (Abu Sufyan) would likely not protect her given his hostility to the Prophet; and she could not remain married to an apostate under Islamic law.
She remained Muslim.
The Negus’s Proxy Marriage
The Prophet, aware of her situation, sent an emissary to the Negus (the Christian king of Abyssinia, Ashama ibn Abjar — who is considered by some Islamic traditions to have converted to Islam secretly) with a marriage proposal to Umm Habiba on the Prophet’s behalf.
The Negus performed the marriage contract — acting as the Prophet’s proxy — with a mahr of 400 dinars from his own treasury as a gift. This is the only case in early Islamic history of the Prophet’s marriage contract being performed by a non-Muslim ruler acting as his representative.
The Family Tension
When Abu Sufyan later came to Medina to renew the peace treaty, he went to his daughter Umm Habiba’s room and attempted to sit on the Prophet’s mat. She reportedly folded it up before he could sit on it, saying: “It is the mat of the Prophet of God, and you are an impure polytheist. I did not want you to sit on it.” Abu Sufyan reportedly left shaken.
See also: Seerah Abu Sufyan, Seerah Khadijah, Seerah Hafsa Bint Umar, Seerah Hijra Abyssinia, Aisha Bint Abi Bakr