42 CFR § 424.55 - Payment to the supplier.
(a) Medicare pays the supplier for covered services if the beneficiary (or the person authorized to request payment on the beneficiary's behalf) assigns the claim to the supplier and the supplier accepts assignment.
(b) In accepting assignment, the supplier agrees to the following:
(1) To accept, as full charge for the service, the amount approved by the carrier as the basis for determining the Medicare Part B payment (the reasonable charge or the lesser of the fee schedule amount and the actual charge).
(2) To limit charges to the beneficiary or any other source as follows:
(i) To collect nothing for those services for which Medicare pays 100 percent of the Medicare approved amount.
(ii) To collect only the difference between the Medicare approved amount and the Medicare Part B payment (for example, the amount of any reduction in incurred expenses under § 410.155(c), any applicable deductible amount, and any applicable coinsurance amount) for services for which Medicare pays less than 100 percent of the approved amount.
(3) Not to charge the beneficiary when Medicare paid for services determined to be “not reasonable or necessary” if -
(c) Exception. In situations when payment under the Act can only be made on an assignment-related basis or when payment is for services furnished by a participating physician or supplier, the beneficiary (or the person authorized to request payment on the beneficiary's behalf) is not required to assign the claim to the supplier in order for an assignment to be effective.
(d) For purposes of claims for services submitted by an MDPP supplier (as defined at § 410.79(b) of this chapter), Medicare deems such claims to have been assigned by the beneficiary (or the person authorized to request payment on the beneficiary's behalf) and the assignment accepted by the MDPP supplier.