The Scriptures are confused and self-contradictory about the marriages of Meirav and Michal, two daughters of King Saul.

One section relates how Saul intended to marry Meirav to David, and in the end married her to another man, Adriel the Meholathite. “But it came to pass at the time when Meirav Saul’s daughter should have been given to David, that she was given to Adriel the Meholathite to wife” (I Samuel 18:19).
A different verse states that Adriel the Meholathite married Michal and not Meirav: “the five sons of Michal the daughter of Saul, whom she bore to Adriel, the son of Barzillai the Meholathite” (II Samuel 21:8).

A different verse describes how David married Michal in exchange for 200 Philistine foreskins. “David and his men went out and killed two hundred Philistines and presented all their foreskins to the king. So Saul gave Michal to David to be his wife” (I Samuel 18:27).
Yet a different verse notes that the payment for David and Michal’s marriage was only 100 Philistine foreskins: ‘David then sent this message to Ishbosheth, Saul’s son: “Give me back my wife Michal, for I bought her with the lives of one hundred Philistines” (II Samuel 3:14).
Another verse states the Michal actually was married to a different man, Palti the son of Laish. “So Ishbosheth took Michal away from her husband Palti son of Laish. Palti followed along behind her as far as Bahurim, weeping as he went” (II Samuel 3:15-16).

One verse states that Michal never had children: “So Michal, the daughter of Saul, remained childless throughout her life” (II Samuel 6:23).
A different verse states that she had five children: “the five sons of Michal the daughter of Saul, whom she bore to Adriel, the son of Barzillai the Meholathite” (II Samuel 21:8).

To understand Chazal’s interpretation of the Scriptures, you must accept two premises:
1. David married both Meirav and Michal.
2. One of the ways to marry a woman is with money or something worth money (as a ring, in our days, or Philistine foreskins). Money or its equivalent must be given to the woman to wed her, and if in place of money he gave her a promissory note, the marriage is not valid. Chazal say of this “one who weds with a loan does not wed.”
This is how Chazal explain the contradictory verses:
The scholars asked Rabbi Yossi: How is it possible that David married two sisters (Meirav and Michal); is it not forbidden to marry two sisters? Answer: David married Michal only after Meirav had died. A different sage, R’ Joshua, gave a different answer. He said that David’s marriage to Meirav was found to be an erroneous sanctification, and so the marriage lost its validity, allowing David to marry Meirav’s sister Michal. How do we know that the sanctification was erroneous and so Meirav had never actually been David’s legal wife? It is written: “Give me my wife Michal” — Michal is my wife, not Meirav, meaning that Meirav was never David’s legal wife. The scholars went on to discuss what the basis for the erroneous sanctification between David and Meirav was. Answer: David married Meirav with a promissory note, and we have already stated that “one who weds with a loan does not wed.” What was the debt which Saul owed David? Answer: Saul promised any who killed Goliath money and wealth, as is written, “the man who kills him, the king will enrich him with great riches” (I Samuel 17:25). After it became clear that David’s marriage to Meirav was invalid, Saul told David that if he was interested in his daughter Michal, he must bring 100 Philistine foreskins as a bride-price. David went and brought Saul 100 Philistine foreskins. With these 100 foreskins and the old debt which Saul owed David, David married Michal. In essence, David said: “Michal is sanctified to me according to the laws of Moses and Israel by the value of the Philistine foreskins and the old debt.” David thought that one who weds using money (the Philistine foreskins) along with a debt intends to marry using the money and not the debt, and so the marriage of David and Michal was valid. Saul, on the other hand, thought that one who uses money and a debt to marry intends to effect the marriage through the debt, and so the marriage is not valid. So according to Saul David’s marriage to Michal was not legal, and he therefore wed her to Palti, son of Laish. Another possibility given by the scholar to explain the disagreement between Saul and David is that Saul thought the Philistine foreskins had no monetary value, so the marriage was not valid. According to David the foreskins did have a monetary value, for dogs and cats could eat them. The scholars went on to another issue: according to David Michal was a married woman, so how could Palti the son of Laish wed her and have sexual intercourse with a married woman? Answer: Palti did not have sexual relations with Michal at any time during their marriage. The scholars asked: Is it not written “And her husband went with her,” implying that he treated her as husbands do wives, and that he had had sexual intercourse with her? No, the scholars replied, he treated her with affection and concern, as a husband treats a wife. The scholars went on to ask: Is it not written “weeping as he went,” implying that Palti thought Michal to be his wife, for otherwise he would not have wept? Answer: Palti cried because he was losing the merit of controlling his urges, for the whole time Michal had been with him he did not have sexual relations with her. Proof that they had not had sexual relations is that it is written “as far as Bahurim [a place name, lit. youths],” for the Scriptures treated them like youths who had not yet had a taste of sexual intercourse.
According to the sage above, Rabbi Yossi, who thought that Meirav and David had been properly married, how did Meirav marry Adriel? Answer: These was, indeed, a sinful marriage, and Meirav and Adriel violated the prohibition against having sexual relations with another man’s wife.
The Scriptures have Michal married to Adriel and give them five children, in contrast to and contradiction of the verse which states Michal had no children and in contradiction of the verse which has Adriel married to Meirav. They reconcile it by stating that Meirav was married to Adriel and they had five children, but Michal raised them. As tribute to Michal’s actions the Scriptures attributed Meirav’s children to Michal, and Meirav’s husband Adriel was listed as Michal’s partner.
(Babylonian Talmud, Tractate Sanhedrin 19b)