Jamieson, Fausset and Brown on 1 Samuel 18:25

25. The king desireth not any dowry--In Eastern countries the husband purchases his wife either by gifts or services. As neither David nor his family were in circumstances to give a suitable dowry for a princess, the king intimated that he would be graciously pleased to accept some gallant deed in the public service.

a hundred foreskins of the Philistines--Such mutilations on the bodies of their slain enemies were commonly practised in ancient war, and the number told indicated the glory of the victory. Saul's willingness to accept a public service had an air of liberality, while his choice of so difficult and hazardous a service seemed only putting a proper value on gaining the hand of a king's daughter. But he covered unprincipled malice against David under this proposal, which exhibited a zeal for God and the covenant of circumcision.