Wa Alaykum Assalam and thank you for your questions.

Women captives were given the choice to accept Islam in order to be manumitted or remain on their religions and stay “captives of right hand possessions”.

When they accepted Islam, the companions, and in this case the Prophet (peace be upon him), married them. So they became their wives and no longer captives.

Her manumission was her dowry because it is a joyful occasion for her; it is something that she was pleased with; a token that replaces material things. She was more in need of that then gold and silver since she regained her honor and status.

Safiya bint Huyai was a Jew but became a Muslim by choice.

I hope this helps answer your questions.

Salam and please keep in touch.