How could Sha'ul give David's wife to another man?
Radak citing Sanhedrin 19b #1: Indeed, Palti ben Layish's Kidushin was b'Aveirah. 1
Radak citing Sanhedrin 19b #2: David's Kidushin of Michal was mistaken. Sha'ul had promised money to the one who will kill Galyus. A loan does not make Kidushin. Even though David also gave 100 foreskins of Plishtim 2 , Sha'ul held that he (the Mekadesh 3 ) intends for Kidushin 4 via the loan, and this is invalid.
Bereishis Rabah (32:1): Do'eg permitted Arayos. He told Sha'ul that David is Mored b'Malchus, so he is considered dead, so she is considered single. (Really, she needs a Get.)
Malbim citing Mahari: Sha'ul did not give her to Palti for a wife, rather, to guard her.
Sanhedrin 19b: They are worth a Perutah, to feed them to dogs and cats. Another opinion there says that they are not worth a Perutah.
The text in Bava Kama 99b says 'she intends for Kidushin through the loan.'
Kesuvos 74a: Some say that if one had Bi'ah after mistaken Kidushin, he intends to be Mekadesh her through Bi'ah, to avoid Bi'as Zenus! (Perhaps this answer holds like the other opinion there. Alternatively, Michal was a minor or Na'arah, so Kidushin required Sha'ul's consent, and he did not consent. - PF)
According to the opinion that Palti's Kidushin was b'Aveirah, how could David remarry her later? He did not remarry her b'Isur - he sinned "Rak bi'Dvar Uriyah" (Melachim 1 15:5)!
Radak citing Sanhedrin 19b: Palti never had Bi'ah with her. Rashba (Teshuvah 1:10) - we must say that a Navi told David. (It is not feasible that witnesses would know this.)
Radak: Sha'ul saw that David often fled, so he forced him to divorce Michal. David told witnesses that he is forced, or he sent it via a Shali'ach and canceled the Shelichus before she received it. She had no way to know that her divorce was invalid, 1 so her marriage to Palti b'Shogeg was like Ones, and she remained permitted to David.
Rashba (Teshuvah 1:10) #1: Michal was Anusah (as if she was forced, i.e. raped) because she thought that David's Kidushin was invalid, like Sha'ul (Malbim - and Beis Din) held. 2
Rashba (Teshuvah 1:10) #2: Michal was Anusah (Sha'ul forced her to have Bi'ah with Palti) and totally passive. Esther was similarly permitted to Mordechai (until she willingly went to Achashverosh).
Refer to 25:44:1:4.
Why would David not tell her, and let her and Palti sin? Perhaps she would not believe him without witnesses, and witnesses were afraid to tell her, lest Sha'ul kill them. Would it not be better to divorce absolutely, to save them from the Aveirah? Perhaps David assumed that she knew that the Get was forced and invalid. Or, he thought that he will be able to tell her before she marries, but he was not able. (PF)
