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

A man acts as his father’s agent, collecting money owed to him by others. When he approached one debtor, the latter said, “I signed it for your father.” He then said, “If you release me and settle with me for a certain amount, that will settle the matter between us.” They then settled a debt of one thousand dirhams for three hundred dirhams. After the settlement, the debtor acknowledged the debt and took possession of the release. May a claim be brought against him after his acknowledgment, when witnesses attest that the creditor released him, or not?

He answered:

An agent authorized to collect a debt cannot validly release the debtor or settle for only part of the amount owed, even if he has been appointed as an agent for that purpose. Moreover, if the debtor denies the debt until a settlement is reached, the settlement is invalid as regards him, and his liability is not discharged. If the claimant agreed to the settlement only for fear of losing the entire amount owed, he was acting under duress, so his settlement is invalid. He may subsequently demand what he is owed if the debtor acknowledges it or evidence establishes it.