Sorry, I was out of town last week.
Medicare secondary payor is not an easy explanation. It seems in my experience that they pay the balance after the primary payment up to the Medicare allowed amount. Unless the primary didn't make payment due to deductible, or non covered service. Then Medicare seems to pay 80% of their allowed amount as if they were prime.
In your example, I would expect Medicare to pay $0 as you stated. I also would expect the Medicare eob to state that the patient responsibility is $0 as well, since the primary insurance paid more than the Medicare allowed amount.
Just when you think you've got it straight, they do it differently!
Michele