42 CFR §424.56

1. (a)Conditions for split payment. If the beneficiary assigns the claim after paying part of the bill, payment may be made partly to the beneficiary and partly to the supplier.
2. (b)Payment to the supplier. Payment to the supplier who submits the assigned claim is for whichever of the following amounts is less:
   1. (1)The reasonable charge minus the amount the beneficiary had already paid to the supplier; or
   2. (2)The full Part B benefit due for the services furnished.
3. (c)Payment to the beneficiary. Any part of the Part B benefit which, on the basis of paragraph (b) of this section, is not payable to the supplier, is paid to the beneficiary.
4. (d)Examples.