Koen,
Some background information will help us - Does your company want to map returns process in SAP? Are you a business user / manager or an SAP analyst?
What is the industry, products and return processes followed in your company?
Returns is one of the standard SD processes. The link by Veselina describes the flow clearly.
You have mentioned some business needs for example customer refunded (less transportation cost), where to keep the received goods; these are handled by functionalities in SAP SD for example copy control, pricing procedure, schedule line categories and movement types.
If you can give some details of your background and requirement, maybe members can address those.
In general, there is lot of document about returns on the internet. Also might help:
Returns - comprehensive analysis
success in interviews - part 1: Process
TW