Saul said, Thus shall you tell David, The king desires no dowry except one hundred foreskins of the Philistines, to be avenged of the king's enemies. Now Saul thought to make David fall by the hand of the Philistines.
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.