QB Issue Resolution:

If you have created a credit memo and want to issue a refund instead you will need to delete the credit and create a refund receipt:

Locate the credit memo or refund.
Go to the Customers.
To highlight click on the customer’s name.
Under Transactions, set Show to All transactions.
Double-click on the credit memo or refund.
On the bottom under More click on the Delete button.

Once the credit memo or refund is deleted, enter in the intended refund.

This article has more detail on issuing refunds that may also be helpful:

Resolution for Issue 'How do I issue a partial refund for a paid invoice?' available: Yes (Solved).
Source: Intuit Community forum.

How do I issue a partial refund for a paid invoice?: 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.

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