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If they clearly accepted the lower payment as full satisfaction of the debt, then the debt balance is now legally $0.
File a formal dispute contesting the continued showing of a balance, and if they verify the accuracy, you can then file a civil action and get it reviewed by the courts.
You must first file a formal dispute via the CRA and get their verificaiton before you are permitted to bring civil action, so get a dispute in the mail and get their determination on the dispute. See FCRA 623(c).