[Acting upon reconciliation or mutual agreement after a judgment has been issued in the case]

The answer, by Allah’s enabling, is:
– That what is to be acted upon is the last reconciliation, with everything it contains, whether it agrees with what came before it or differs from it.
– That is because each of the two disputants has been content and satisfied with what has come to him in the last reconciliation, and is satisfied that his opponent should have what has come to him, and has completely yielded that to him and to every claim regarding it, and has fully relinquished it.
– Moreover, the reconciliation between the two disputants concerning a matter is considered an acknowledgement from each of them to the other of ownership of that which has come to him by this last reconciliation.
If it is said: Suppose that after this last reconciliation one of the two disputants claims that he only discovered the document of the previous judgment or the previous reconciliation after this last reconciliation, and that had he known of it before the last reconciliation he would not have been satisfied with it – does that constitute an excuse for him to annul the last reconciliation?
We say: That is not a valid justification for annulling the last reconciliation, for the reasons we have already mentioned: that reconciliation is, in essence,
1. The renunciation, by each of the two disputants, of every claim concerning what has come to his opponent;
2. The acknowledgement, by each of them, that the other owns what has come to him by that reconciliation.
It is not valid for either of them to demand what he has yielded to his opponent or what he has acknowledged to be his.
What we have mentioned is further supported – in addition to what has preceded – by what is narrated from the Commander of the Believers (Peace be upon Him): that a man came to him and said: “O Commander of the Believers, my slave has married without my permission.” So the Commander of the Believers (Peace be upon Him) said: “Separate between them.” The man then said to his slave: “Divorce her, O enemy of Allah!” So the Commander of the Believers (Peace be upon Him) said to the slave: “Now, O slave, if you wish you may keep her, and if you wish you may divorce her.” The man objected to the Commander of the Believers (Peace be upon Him) because he had taken the matter out of his hand and placed it in the hand of the slave. So the Commander of the Believers (Peace be upon Him) clarified to him what he objected to and said: “You have confirmed him in the marriage and given him permission for it when you commanded him to divorce her, for you would not have commanded him to divorce her except that you had acknowledged the contract of marriage.” This is the meaning of the narration, not its exact wording – so let it be known.
The words of the Commander of the Believers (Peace be upon Him) in this narration indicate that a person’s yielding his right to another person is a route for the right to be established for that other person and for the first person’s right to be cut off, and that no claim is to be heard thereafter from the one who has yielded.
– And the narration indicates that it is obligatory upon the judge to rule according to what is outward, even if the one against whom he rules is heedless or naïve.
Similar to what has preceded is when there are dealings of debt between two men. Then the records of the debt are lost or stolen, and the two men wish to free themselves from that debt. They meet, estimate the debts, and negotiate concerning them, and their opinion agrees that the debtor shall pay a specific sum to his companion, and that they shall mutually forgive what is more or less than that. The payer then yields what may be in excess of that amount of debt, if there is any; and the receiver yields what may be less of his debt, if there is any. They separate on that basis, with a document written by an upright scribe and two upright witnesses. Then, after some time, the lost records are found. Does the finding of those records constitute a reason to annul that reconciliation, mutual relinquishment and mutual pardon, or not?
– The answer is as we have already mentioned in the first question, namely that reconciliation and mutual agreement have cut off the path of any further claims in that matter.
Yes, if the reconciliation and relinquishment were conditioned upon the non-appearance of the record, then in that case the record is acted upon.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2