Greetings from the Community, @Vera.

Thanks for bringing this matter to my attention. To clarify, does the Invoice transaction within QuickBooks still show as having a payment attached? Follow the steps below to verify this:

1. From QuickBooks Online, navigate to the Sales tab. Select Customers.
2. Click on the customer who you'll be re-sending the invoice to.
3. Find the invoice and check the Status column to see if it shows Paid, Open, or Overdue. Click to open it.
4. Verify if there's a Payment status in the top-right corner. If there's a payment applied, you can click the link to open it and un-check the invoice to remove it.

The attached video shows the process with an invoice that has been paid:

Once you've removed the payment, try opening the invoice and selecting Save and send at the bottom to see if the balance will show to your customer. In the interest of time, it's also possible for them to make a payment even though the balance is showing $0. Check out our guide on recording customer payments to learn more.

This will help you to clear that invoice's balance and recollect the customer payment. Be sure to touch base with here on your results with this, I want to make sure you're taken care of. Thanks again and have a great rest of your day.