Anhrah,
You are talking about Hazrat Zainab (ra) and her marriage to AnHazoor (saw). Let me give you a little history lesson.
Hazrat Zainab (ra) was from a rich family well known among of the Arabs for their status and wealth. When she and her brother asked AnHazoor (saw) for advice in the matter of her marriage, the Holy Prophet (saw) advised to marry Hazrat Zaid (ra) who was the Holy Prophet's called/adopted son! Both Hazrat Zainab (ra) and her brother did not like the idea because Hazrat Zaid (ra) was a free slave and had no other worldly status. AnHazoor (saw) asked them to fear God and not consider worldly status the only reason for rejecting a proposal.
So Hazrat Zainab (ra) with her own will agreed to marry Hazrat Ziad (ra) and most part of it was her obedience to the Holy Prophet (saw). According to some traditions the Holy Prophet (saw) gave her dowry as a show of affection towards Hazrat Zaid (as) and as his blessing for the couple in general.
Later things didn't work out between the two. There are clear historical proof that Hazrat Zaid (ra) came to the Holy prophet (saw) unhappy about his marriage and wanting a divorce. AnHazoor (saw) argued against it and told him repeatedly to try to work out the marriage. Though when things get worse, Hazrat Zaid (ra) divorced Hazrat Zainab (ra).
After that, due to Hazrat Zainab's (ra) sacrifice of her wishes, along with her Worldy status, just to obey the Holy Prophet (saw) in marrying Hazrat Zaid (as) and the hurt she went through as the marriage didn't work out, he married her himself. And gave her the highest respectable status of 'Omoul-Momeneen"-the Mother of the righteous!! And to that marriage too, she agreed and accepted at her own will!
p.s: Stop reading the hate sites, they make a dog out of a good Brahmin.