The days were not expired - David was so rapid in his attack upon the Philistines that he was able to bring the required dowry within the time, and to receive his wife (Michal), before the time had expired within which he was to receive Merab.
Related Commentaries of 1 Samuel 18:26
1 Samuel 18:26
26 And when his servants told David these words, it pleased David well to be the king's son in law: and the days were not expired.e