Hi Manoj,
HI their is no Replacement car issue process in SAP DBM in first place so no question of returning it 
. Appointment is just to get an idea of availability of replacement cars. Nothing more. Instead if you really want this process then you have to do custom actions (very simple, no coding required), in VMS secondary action matrix .
Create 2 simple actions "check out" and "check in" and assign it to secondary matrix action control and use it for replacement vehicles. So after vehicle booked in using appointment they can do check-out action and after receiving back they can execute check in action.
Regards
Ganesh