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The company should be giving you credit for this based on the receipt of funds from the court.
(It is not "illegal" in the sense that they are breaking the law, but they are not entitled to collect interest on something they are no longer owed, so you may need to go back to court for an "ex parte hearing" to get a redetermination (also called a "recalculation") of your debt).
Frankly, courts are not going to be terribly pleased about having to deal with this (not with you, they will be upset with the creditor, the creditor is wasting the court's time and your time). Contact the creditor again (in writing), and let them know you plan on going to court to get the judgment amount recalcluated if they do not refigure the amount and confirm it with you in writing within a short time (for example "10 days from the date of this letter").