CREDIT NOTE

hello Sir, this is a real scenario at work, please advice on how to treat this we have sales from 2013 to 2016, the customer has not paid and we analysed the invoices and found out that some amounts on invoices were incorrectly invoiced but they are arising from past years, my question is should we pass the credit notes or we should consider them as bad debts, since we have been reporting them in our financials. note also that this figures are material.