These can be either done at month-end or year-end.
Either way, they are processed in the same way and against each individual stock record.
Go into the stock record click on View/post tab at the top and then select write down. A box pops up as below:-
Enter the amount you want to write the vehicle back by, enter the date in which you want the write back to go through for. Lastly either leave the nominal code blank and this will go through automatically as an expense to car sales or you can enter a nominal code and charge to another department.
For example 18.104.22.168 Loan Car Costs being charged to workshop.