How are the twenty qirats that the man wrote to his deceased wife to be distributed, and is it permissible for him to retract their donation after her death?

Whatever qararit (land shares) are purchased with the wife's money, they belong to her during her lifetime and to her heirs after her death. Similarly, if she gifts her money to her husband in exchange for its equivalent in land, he has no right to revoke it. This is known as a gift of reward (hibat al-thawab).

As for what is purchased with the husband's money and then gifted by him to his wife, it does not belong to the wife and her heirs unless the conditions for the validity of the gift are met, the most important of which is the possession and قبض (seizing) of the gift during the lifetime of the giver. In this case, the gifted land belongs to the wife's heirs, and the husband has no right to it except his share of the inheritance.