One who contracts marriage unconditionally with no stipulation in it, the marriage stands even if their intention, or the intention of one of them, was to make her lawful for her former husband

Imam al-Shafi'i (may Allah have mercy on him) said: "Because intention is something said within the self, and people have been excused (from being taken to account) for what they say within themselves."

Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Allah has pardoned my Ummah for the whisperings that arise in a person's heart and mind, as long as he does not speak them with his tongue or act upon them."
(b) In the narration of Hisham, the Prophet (peace and blessings be upon him) said: "Allah has pardoned my Ummah."

Mujahid (may Allah have mercy on him) said: A man of Quraysh gave his wife an irrevocable divorce (talaq battah). Then he passed by an old Bedouin man and his son in the market, who had come there for trade. He said to the young man: Is there any good in you? Then he went away from him, then came back to him and said the same, then went away again. He said: Yes. He said: Show me your hand. Then he went with him, told him the matter and instructed him to marry that woman. He married her and spent the night with her. When morning came, he asked permission (to leave) and was given permission; then suddenly he had turned his back on her. The woman said (to the first husband): If he divorces me now, I will never marry you. This was mentioned to Sayyiduna Umar (may Allah be pleased with him), so he summoned him and said: Had you married her, I would have done such and such to you, and he threatened him. And he summoned her husband and said: Hold on to her. In another place it adds that he said: If anyone approaches you with anything, inform me.

Ibn Sirin (may Allah have mercy on him) said: A woman's husband divorced her three times. There was a poor Bedouin who used to sit at the door of the mosque. A woman came to him and said: Would you like a woman whom you marry, spend the night with, and then leave in the morning? He said: Yes. So that is what happened. Then his (new) wife said to him: When morning comes, they will tell you to leave her; do not do it, for I will provide for you what you see, and go to Sayyiduna Umar (may Allah be pleased with him). When morning came, they came to him and they came to her. She said: Speak to him; you are the ones who brought him. They spoke to him, but he refused, and he went to Sayyiduna Umar (may Allah be pleased with him), who said: Keep your wife, and if they trouble you with anything suspicious, come to me. And he sent for the woman who had gone about arranging this and punished her as an example. After that the poor man used to come to Sayyiduna Umar (may Allah be pleased with him) morning and evening wearing a fine garment (hullah), and he would say: Praise be to Allah who has clothed you, O man of the two patches, in a garment in which you come and go morning and evening.
Imam al-Shafi'i (may Allah have mercy on him) said: I have also heard this hadith from Ibn Sirin as a musnad, shadhdh, connected report; he connects it to Sayyiduna Umar (may Allah be pleased with him) with a similar meaning.