Need to generate an automatic creditmemo for transactions

We shipped the materials to customers by lot wise method and so per day we generated more than 20 invoices(1000 or 2000 qty) as per customers requirement. Some times customer doesn't accept the invoices for some other reasons after 1 week of invoice generation. So we need to cancel or return those invoices. For RMA, We need to raise rma sales order and create a receipt for those return transactions, Then it will create a creditmemo in AR. We need the option for automatic creditmemo generation method. We unnecessary waste the time to create the RMA order and