Details
Description
I can now recreate this bug using Auth.net in TEST mode and submitting 'Credit Card Event Registration' in back-end.
However, I tried applying the patch from this PR (https://github.com/civicrm/civicrm-core/pull/4228) and it throws a fatal DB error, so the fix is not quite correct (although I think the theory that the problem is caused by fee_amount param having 2 'meanings').
To recreate, you'll need to use Auth.net test credentials (which I will forward to you). Then you'll need to patch CRM_Core_Payment_AuthorizeNet using the attached patch which forces a fee to be recorded (normally TEST mode doesn't pass back a fee).
Then setup Auth.net as your default payment processor and do back-office event registration. You'll see that civicrm_participant.fee_amount is set to 1.50 (the auth.net fee) instead of the actual fee for the event.