I need to apply ONE specific vendor credit to a One specific invoice

Not to worry, Clara. I'm determined to help you apply vendor credit to one specific invoice.

The best way to handle this in QuickBooks Online is to use a clearing account to help transfer or apply a vendor credit to the customer's invoice. I'll show you how:

1. Go to the Gear icon and then select Chart of Accounts.
2. Click the New tab.
3. Choose Bank for the Account Type. 
4. Type in the Name for the clearing account.
5. Make sure you don't add an amount to the balance, then click Save and Close.

Then, let's create two journal entries. Here's how:

1. Go to + New and then select Journal entry.
2. Debit the Clearing account (created in Step 1) and credit Accounts Receivable.
3. In the Name field on the Accounts Receivable row, enter the customer name.
4. Add the other necessary details, then select Save and new.
5. Create a second general entry. This time, debit Accounts Payable and credit Clearing Account for the amount of the sale.
6. In the Name field on the Accounts Payable row, enter the vendor name that needs the bill credit created.
7. Click Save & Close.

After that, let's link the journal entries to the specific vendor credit and invoice:

1. Go to the + New and select Pay bills.
2. Link the journal entry and vendor credit.
3. Click Save and close.
4. Once done, go to the Receive Payments window.
5. From there, link the journal entry and invoice.
6. Select Save and close.

Additionally, I've added these articles that'll help you record a refund from a vendor or if you need to return your customer's money:

• Enter Vendor Credits and Refunds
• Record a Customer Refund in QuickBooks Online

Please keep me posted if you need additional assistance tracking the transactions in QuickBooks Online. I want to ensure you're all set.