I think it's that the service provided for the $8,ooo charge the provider says it costs, is an accepted service.
So the 80/20 will apply, after the provider adjusts the price.
And therefore, Medicare pays 80%, according to what the Medicare fee scale shows for that service, (far less than the providers original price quoted)
The Provider makes an "adjustment" which is a credit sort-of, and is subtracted from the whole of your original bill. That is what they agreed to do, when they "accepted assignment."
Then it comes out to your paying the 20% (of the Medicare-approved amount.)
The 80/20 applies because it's a covered service.
but the provider price is reduced, because the provider accepts assignment.
The best thing to do, whenever you are in doubt, or in general, is to wait until you get, in the paper mail:
an explanation from Medicare itself, which will show exactly how much that Medicare was billed by each one of your providers, and whether Medicare approves the service, and how much Medicare paid if it did approve the service,
and then, that same paper, says:
What that provider is allowed to bill you.
(That is the max you should pay your provider. Not more.)
This paper statement from Medicare, usually gets to you, in your mail, within 2 weeks or so, from when they received the provider's bill, so it is not long to wait, and a Provider should be willing to wait till you see that, before you pay them your part of the balance.
The provider will already have received Medicare's notice, and will have received payment from Medicare,
unless A) it is not a covered service, or B) you haven't met your yearly deductible yet.
If A; then you might have to pay it to the Provider, but you might not, if you were not given info that it likely would not be a covered service by Medicare.
If B; then, then your amount to pay would still be reduced, but you will have to pay the deductible total to all your providers, before Medicare actually pays their 80%.
Be sure to let the Provider bill Medicare, for everything, so that the charges WILL count toward your deductible.
This occurs every year in January.
I hope this helps.
I am not an expert, but have some experience with this topic.