The Law Firm’s advocates and attorneys successfully represented a Client in a case for payment for the return of undue payment, fulfilled by the Client in favor of an entity that, as a result of the assignment agreement, ceased to be a creditor. In the circumstances of the case in question, the Law Firm’s Client, as a result of a mistake, despite receiving notice of assignment of receivables, paid to the seller of receivables, rather than to the buyer of receivables. The buyer of the receivables, as a result of receiving the writ of execution against the Law Firm’s Client, initiated effective enforcement and enforced the amounts arising from the court decision. Under these facts, the law firm’s Client decided to file a lawsuit against the debt buyer for the return of the amount paid to him. The court proceeding in the present case agreed with the argumentation of the Law Firm’s lawyers that the financial benefit obtained by the seller of the debt at a time when he was no longer a creditor has the characteristics of unjust enrichment, and thus there are grounds for demanding the return of the fulfilled benefit as undue. Thanks to quick and effective actions of the Law Firm’s lawyers, the case ended with the Law Firm’s Client obtaining a judgment awarding him the return of the undue payment and full enforcement of the amounts awarded to him.