If you are a business owner or accountant using QuickBooks Online, you may need to know how to apply a credit memo to an invoice in order to properly manage your finances. A credit memo is a document that is issued by a seller to a buyer, reducing the amount owed on an invoice. Applying a credit memo to an invoice in QuickBooks Online is a simple process that can help you keep track of your finances and ensure that you are accurately accounting for all of your transactions.