Return order for item which is Service item ,which is OE Transactable (NO) & Stockable (No)

Dear All

Return order is need to book for item which is service type & item is Not Stockable & OE Transactable is also No , We need to book RMA &  once is  order is booked Credit memo to be generated , which is same as invoice reference taken in RMA order,

As far as usually when item is taken in inventory against RMA order for Inventory , & Auto invoice is run Credit memo is generated

I need to generate the credit memo for non Stockable & OE Transactable (NO) please advise which line flow is required & how to do the same

Regard

Naresh Singhal