Returns order with reference to Cash Sales invoice

Hi Gurus,

our client have requirement to make a return order w.r.t a billing document.

How to map the process and then post credit memo with reference to the return order.

the transaction processed as below.

cash service order > billing document.

the requirement is as below.

Billing document > return order >credit memo request posting.

appreciate help....

Regards,

Mehmood