Hi Rudra,
I you are confused then how you will propose a solution to your client
.
Ideally you should had explore both the module and done some homework then probably you problem should have been solved.
Anyway looking into you business process and kind of business your client is you need to implement REFX, REFX don't have any integration with SD, it is very closely integrated FI. SAP RE will not have much use in you business process.
Please explore REFX before getting into solutioning part.
Many Thanks
SS