He—may Allah Most High have mercy on him—was asked:

A woman released her husband from his obligation to pay her entire dowry. Afterwards, the husband had witnesses attest that he had divorced his wife in exchange for that release, although the release had already taken place. Is the divorce valid? If it takes effect, is it revocable or not?

He replied:

If they had agreed that she would give up her dowry and release him from his obligation in exchange for his divorcing her, and she then released him and he divorced her, that constitutes an irrevocable divorce. The same applies if he said to her, “Release me from my obligation, and I will divorce you,” or, “If you release me from my obligation, I will divorce you,” or used similar expressions, whether those of the learned or ordinary people, which convey that he requested the release in exchange for divorcing her.

If, however, she released him from his obligation independently of any divorce, and he subsequently divorced her, the divorce is revocable. But may she retract the release, where doing so is possible, on the grounds that such a release is ordinarily granted only so that he will keep her as his wife, or out of fear that he will divorce her, take another wife, or the like? There are two opinions on this, both reported from Ahmad.

If, however, she willingly granted the release unconditionally—that is, she initiated it herself, without anything on his part prompting it and without receiving anything in return—then she unquestionably has no right to retract it. And Allah knows best.