QB Issue Resolution:

When an invoice is marked as paid, it creates a payment transaction. From the payment transaction you can remove the payment.

Go to Customers (left navigation bar) > [Customer Name]
Click the drop down menu next to Show > Select All Transactions
Click Payment > Clear Payment > Save

Resolution for Issue 'How to deal with declined credit card payments which mark invoices as PAID?' available: Yes (Solved).
Source: Intuit Community forum.

How to deal with declined credit card payments which mark invoices as PAID?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for Credit Card Processing
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