42 CFR §411.32

1. (a)Basic rules.
   1. (1)Medicare benefits are secondary to benefits payable by a primary payer even if State law or the primary payer states that its benefits are secondary to Medicare benefits or otherwise limits its payments to Medicare beneficiaries.
   2. (2)Except as provided in paragraph (b) of this section, Medicare makes secondary payments, within the limits specified in paragraph (c) of this section and in § 411.33, to supplement the primary payment if that payment is less than the charges for the services and, in the case of services paid on other than a reasonable charge basis, less than the gross amount payable by Medicare under § 411.33(e).
2. (b)Exception. Medicare does not make a secondary payment if the provider or supplier is either obligated to accept, or voluntarily accepts, as full payment, a primary payment that is less than its charges.
3. (c)General limitation: Failure to file a proper claim. When a provider or supplier, or a beneficiary who is not physically or mentally incapacitated, receives a reduced primary payment because of failure to file a proper claim, the Medicare secondary payment may not exceed the amount that would have been payable under § 411.33 if the primary payer had paid on the basis of a proper claim.