Third Party Payor For Medicare Supplement

I enrolled my client into a medicare supplement through Manhattan Life. Her employer wants to pay them directly however they said that they do not accept third party payors. Her employer won't provide her with the funds to make the payment through her checking account because they stated it would constitute as a raise. I called Cigna and they do not accept third party payors either. Is there an insurance company that accepts third party payors for their medicare supplement clients?

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