Must medical providers initiate a refund when they charge a customer more than the insurance-adjusted amount?

Question

Assume an individual in the United States went to a medical provider and got charged at the end of the visit, by the provider's billing staff. The billing staff knew that this visit was partially covered by insurance, and the billing staff determined that the amount owing to the provider would be $X. The individual paid $X.

A few weeks later, the claim is processed by the individual's medical insurance, and the insurer's summary indicates that the insurer ended up covering more of the cost than the billing staff had assumed. The individual did not need to pay the provider $X, but instead, something less than $X.

Must medical providers initiate a refund when they charge a customer more than the insurance-adjusted amount? Or is it up to the individual to realize that he got overcharged at the desk and ask for a refund?

I am mostly interested in California and Washington state.

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For an example as to how an individual's share is computed, see the following.

Answer 1

They’re not entitled to keep it, but you do have to ask

It’s important to remember Hanlon’s razor in these circumstances: never attribute to malice that which can be adequately explained by stupidity. That is, a business that doesn’t immediately recognise and refund an overpayment is more likely incompetent than dishonest.

That said, while the money belongs to you, they are under no legal obligation to reach out to you to return it. Just like they would have no obligation to reach out to you if you left your hat in the waiting room. In both cases, they should keep it safe for you but if you don’t ask for its return within the statute of limitations, they can then keep it.