Tosefta su II Samuele 21:8

Similarly, you can say (2 Sam. 21:8), "And the five sons of Michal daughter of Saul that she bore to Adriel [son of Barzilai] the Mecholatite, etc." Here we find that [Saul] gave Michal to Adriel the Mecholatite. But did he not give her, rather, to Palti son of Laish from Gallim? For it says (1 Sam. 25:44), "And Saul gave Michal his daughter, the wife of David, to Palti son of Laish." Rather, the marriage of Michal is juxtaposed to the marriage of Merav (see 1 Sam. 18:19). Just as the marriage of Merav [was consummated] through sin, so too the marriage of Michal to Palti ben Laish [was consummated] through sin.

The students of Rabbi Yosei asked, How could David have married the sister of his wife (i.e., in apparent violation of Torah law, see Lev. 18:18)? He said to them, it was [only] after the death of Merav that he married [Michal]. Rabbi Yehoshua ben Karcha says, [the reason is that] their betrothal was not fully valid, as it is said (2 Sam. 3:14), "[And David sent messengers to Ish-boshet son of Saul, saying,] 'Give me my wife, Michal, for whom I paid the marriage-price [of one hundred Philistine foreskins].'" Just as their betrothal was not fully valid^*the implication is that one hundred foreskins are worthless, and thus do not effect a valid kiddushin (see Kidd. 1:1), so too their marriage was not fully valid. [Relatedly,] on one [place] it says (2 Sam. 21:8), "And the five sons of Michal daughter of Saul that she bore to Adriel [son of Barzilai] the Mecholatite," and [elsewhere] it is written (2 Sam. 6:23), "And Michal daughter of Saul had no offspring until the day of her death." How could these two [apparently contradictory] verses remain standing? It was said, until now, they were [considered] the sons of Merav. Merav bore them, but Michal raised them, and [therefore] they were called by her name. As it says (Ruth 4:17, regarding Ruth's son Oved), "And the neighbors gave him a name, saying, 'A son is born to Naomi,'" and it says (Num. 3:1, prior to listing Aaron's descendants), "And these are the generations of Moses, etc."