What i must do as salesman when a customer return something in case manager in the store or his absence ?

Hi

The Customer is Always Right. Customers return purchased goods when they are unsatisfied with the items (most of the time).  Most of the retail outlets do have a Return Policy and if so there shouldn't be a problem.  

Always check the item and the receipts in front of the customer before you accept the returned good and most importantly document it in a log book with a copy of the receipt.