I would though be careful about billing the patients. Assuming the provider was PAR with Medicare, and if you can bill retroactively, I would check with Medicare to see if you MUST bill the patients the 20%. The law says yes but I wonder if there are extenuating circumstances as in this case. Sending a patient a bill for services that were rendered 11 months ago may impact the relationship with the provider. I am pretty sure you have to bill the patient the 20% even in this case but there is no harm in asking.