I appreciate you for posting your concern here in the Community. Let me help guide you on removing the payment linked to an invoice in QuickBooks Online (QBO).
The invoice will only be marked as paid once there's a payment or deposit applied to the transaction. Let's pull up the Invoice and Received Payments report to verify it.
Once you locate the payment, you can remove the payment linked to an invoice to mark the status as open again.
To do this, please follow the steps below:
For your reference, you may want to check these related articles below and to learn more about recording customer's payment:
That should do it! Feel free to leave a comment below if you have additional concern about invoice payment. I'll be around to help.