“Give me my wife Michal.” This seems cruel since David had other wives and Michal seemed to be happily with the man Phaltiel, but in the culture, once David became king over Israel he could not afford to have a wife (there had been no divorce) with any other man in the kingdom because if she had a child people could claim it was David’s and set up a rival to the throne. David knew this and so said he would not meet with Abner unless Michal was returned to him. Along with that, David’s being reunited with Michal reconnected him to the house of Saul and thus in one way legitimatized David’s rule over the kingdom of Israel once ruled by Saul.
The Law of Moses forbade a man from marrying a woman, divorcing her, then remarrying her (Deut. 24:1-4). Since David does generally obey the Mosaic Law, the fact that David reunites with Michal is a strong indication that he never divorced her, but her father, king Saul, took her from David when he had the chance when David had to flee for his life (1 Sam. 19:14-18). Saul gave Michal to Phaltiel, but she was not legally divorced from David at the time, a point specifically made in Scripture (1 Sam. 25:44). But at that time in his life Saul was ignoring and defying the Word of God in many different ways, and that was just one more way that Saul disobeyed God.