42 CFR § 447.15 - Acceptance of State payment as payment in full.
A State plan must provide that the Medicaid agency must limit participation in the Medicaid program to providers who accept, as payment in full, the amounts paid by the agency plus any deductible, coinsurance or copayment required by the plan to be paid by the individual. The provider may only deny services to any eligible individual on account of the individual's inability to pay the cost sharing amount imposed by the plan in accordance with § 447.52(e). The previous sentence does not apply to an individual who is able to pay. An individual's inability to pay does not eliminate his or her liability for the cost sharing charge.
The following state regulations pages link to this page.