How can I pay an open vendor invoice with an existing debit in a customer account?

Thanks for inquiring about handling transactions when a person acts as both a vendor and a customer in QuickBooks Online, Elke. I'll provide some guidance on how to handle this type of situation. 

For now, there's no direct way to apply a credit or debit from a customer's profile to an open bill where they are listed as the vendor. 

However, there are alternative approaches and workarounds to achieve the desired outcome.

To start, you can record a vendor credit and use it to close out the vendor's invoice. Let me show you how:

1. Click the + New button and select Vendor credit.
2. Choose the name of the payee in the Vendor field.
3. Proceed to the Category column and look for Accounts Payable. 
4. Select Save and close.

After that, follow these steps on how to apply the credit to an open bill: 

1. Go back to the + New button and select Pay bills.
2. Select a bill for your vendor from the list. You'll see the available credit with this vendor in the Credit Applied field.
3. Complete the rest of the fields like you normally do.
4. Click Save and close. 

Alternatively, you can also record a barter transaction or use a clearing account. Check out these articles for guidance about these options: 

• Record A Barter Transaction
• Set Up A Clearing Account

 Although, I would still recommend reaching out to an accountant for additional advice on this matter. This is to make sure that everything adheres to relevant accounting standards and tax regulations.

Allow me to share a couple more articles for additional references in case you might need them: 

• Manage Outstanding Balances For Customers And Vendors In QuickBooks Online
• Run A Report With Vendor Totals

Let me know if you need more help from the Community when paying bills and managing any other transactions in QBO. We're here to guide you all the time.