During User Acceptance Testing (UAT) a tester submits an incident because the Invoice total did...

Re-write the test case with a corrected expected result

Validate the tester is properly following the UAT plan and is trained

Investigate for root cause, if the design was implemented incorrectly then apply corrective action to resolve the defect.

Determine if the expected result is not a request for new functionality.

Adjust formula fields until the correct values match his expectations to provide an immediate fix