42 C.F.R. § 411.31 : Public Health — CENTERS FOR MEDICARE & MEDICAID SERVICES, DEPARTMENT OF HEALTH AND HUMAN SERVICES — MEDICARE PROGRAM — EXCLUSIONS FROM MEDICARE AND LIMITATIONS ON MEDICARE PAYMENT — INSURANCE COVERAGE THAT LIMITS MEDICARE PAYMENT: GENERAL PROVISIONS — Authority to bill primary payers for full charges

(a) The fact that Medicare payments are limited to the DRG amount, or the reasonable charge, reasonable cost, capitation or fee schedule rate, does not affect the amount that a primary payer may pay.
(b) With respect to workers' compensation plans, no-fault insurers, and employer group health plans, a provider or supplier may bill its full charges and expect those charges to be paid unless there are limits imposed by laws other than title XVIII of the Act or by agreements with the primary payer.

42 C.F.R. §411.31