The Cow - 2:237
If you have divorced them after you have paid the bride price, but before you have touched them, you should give them half of the promised bride price. Unless the wife renounces her right or the husband wants to give the full amount. It is more virtuous for you to give it up. Remember the friendship between you. Allah is All-Seeing of what you do.
Related verse:
The Women/4:20
If you want to leave your present wife and marry another wife, do not take anything back from her, even if you have given a large sum to the first one. Will you take it back by fraud and slander and sin?
The Women/4:21
How can you take it back when you were once each other´s closest friends and they took a firm promise from you?