Don’t mess with the completed transaction.  Just pay the customer and use your refunds income account as the expense for the payment

Resolution for 'completed transaction issue: 'How do I create a partial refund?' available: Yes.

The completed transaction 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 completed transaction issue yourself by looking for a resolution described in the article. If it is a complex completed transaction issue or you are unable to solve the issue, you may contact us by clicking here or by using other completed transaction support options.

For more information about resolving issues related to completed transaction in Quickbooks/QBO or to get support for the issue 'How do I create a partial refund?', click the link below: