He Paid a Part of the Dowry, Then He Left
Question :
One of my relatives proposed marriage to my sister and he paid part of the dowry. Then after a year, he left her and married another. What is the ruling on this sum which he paid as a part of the dowry? Is it his right or is it my sister's?
Answer:
He paid this sum to you by his free will and his choice, then he left it to you after rejecting marriage with you. This proves that he has remitted it to you, so you are entitled to it in return for the proposal and the acceptance, and in return for this period of time during which he kept your daughter restrained (from marriage), and because he has left it, and not sought its return. However, if he asks for it, you are free to return it or a part of it, according to your agreement. And Allah knows best.
Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 5 Page 308