C. F. Keil and Franz Delitzsch on 1 Samuel 18:20-21

1Sa 18:20-21 Michal is married to David. - The pretext under which Saul broke his promise is not given, but it appears to have been, at any rate in part, that Merab had no love to David. This may be inferred from 1Sa 18:17, 1Sa 18:18, compared with 1Sa 18:20. Michal, the younger daughter of Saul, loved David. When Saul was told this, the thing was quite right in his eyes. He said, “I will give her to him, that she may become a snare to him, and the hand of the Philistines may come upon him” (sc., if he tries to get the price which I shall require a dowry; cf. 1Sa 18:25). He therefore said to David, “In a second way (בּשׁתּים, as in Job 33:14) shalt thou become my son-in-law.” Saul said this casually to David; but he made no reply, because he had found out the fickleness of Saul, and therefore put no further trust in his words.