Didn't David already have the right or privilege of marriage to Merab as a result of his victory over Goliath?
1 Samuel 18:17 (ESV)
17Then Saul said to David, “Here is my elder daughter Merab. I will give her to you for a wife. Only be valiant for me and fight the LORD’s battles.” For Saul thought, “Let not my hand be against him, but let the hand of the Philistines be against him.”
Saul certainly appeared to have added extra conditions to what was said in 1 Samuel 17:25, before David’s marriage with Merab could take place.
17 Then Saul said to David, “Here is my elder daughter Merab. I will give her to you for a wife. Only be valiant for me and fight the LORD’s battles.” For Saul thought, “Let not my hand be against him, but let the hand of the Philistines be against him.”