Thanks for getting back to this thread, @Rose81.
I'm here to show you other ways to connect an old vendor bill to an old payment in QuickBooks Online (QBO).
First, may I know how you record the bill payment in QuickBooks? If you created a check affecting the AP account, you can easily link it to the open bill through the pay bill. However, if you record it differently, please share some information on how you do it in QuickBooks.
Also, please keep in mind that this can affect your financial statements, so it's best to consult with your accountant before making any modifications.
If you'd like to learn how to give your customers or suppliers their money back, you can read this article: Record a customer refund or supplier refund in QuickBooks Online.
Furthermore, QBO offers dozens of reports to help you obtain an overview of your business. This way, you'll be able to see how your sales and expenses are performing.
Please let me know if there is anything else I can assist you with, @Rose81 and I will be more than happy to help. Stay safe!
