Can a supplier also be a customer account on QB?

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Customers and suppliers post to separate default debtors and creditors account in QuickBooks Online, so you'd need to create a separate customer and supplier for this. If you're needing to record a contra payment between the two accounts, you can then do this using a journal entry, selecting the debtor's account (under 'account') and customer (under 'name') on one line, and the creditors (under 'account') and supplier (under 'name') on the second line. 🙂