The error suggests that there is something on the cash refund entry form (presumably a customization) putting a "<script>" tag or other JS content in the data that gets posted to NetSuite upon submission, and that this same "<script>" tag or scripting content is found in the page later displaying the saved cash sale. If this is indeed the issue, then you should find that the cash refund has actually been saved correctly, and that you can navigate to it... Is that the case?