42 CFR § 422.216(a)(3-4)

(3) Noncontract providers. The organization pays for services of noncontract providers in accordance with Sec. 422.100(b)(2).

(4) Service furnished by providers of service. Any provider of services as defined in Sec. 1861(u) of the Act that does not have in effect a contract establishing payment amounts for services furnished to a beneficiary enrolled in an MA private fee- for- service plan must accept as payment in full the amounts (less any payments under Sec. 412.105(g) and 413.76 of this chapter) that it could collect if the beneficiary were enrolled in original Medicare. (Emphasis added.)