Saul said, I will give her to him, that she may be a snare to him, and that the hand of the Philistines may be against him. Therefore Saul said to David, You shall this day be my son-in-law a second time.
He set up a dowry regarding the Philistines. David went out and gave him double dowry. So Saul then of course was sort of shocked and surprised. He figured David would get wiped out in going out against the Philistines.