The part that concerns me is "They don't know what date it should be but the know the date submitted the patient was not seen."
How can the provider not be able to figure out what the correct date was? This is really key in how to handle this. If the patient wasn't seen at all, you would notify them that the patient was billed in error. But if it's an incorrect date, then you would notify them of that. Two different scenarios that you can't decide which is correct without knowing.
Michele