Saul said to David, Behold, my elder daughter Merab, her will I give you as wife: only be valiant for me, and fight Yahweh's battles. For Saul said, Don't let my hand be on him, but let the hand of the Philistines be on him.
We do not know why Saul failed to honor his promise of giving David his daughter Merab.  It might have been because David (as he did later in regard to the dowry for Michal) indicated that he was unable to provide a suitable dowry.  Certainly, there must have been some good reason for Saul's not honoring a promise that was known to the whole kingdom, namely, that the victor over Goliath would receive the king's daughter in marriage. It was not actually "a dowry," but the present which the bridegroom was supposed to give to the bride's father.