Return Sales Order Automation

Hi All

My requirement is to automate the sales process ie: once sales order is created, Return Outbound delivery should be created followed by Post Goods Receipt and finally Invoice creation. I have used BAPI for create outbound delivery, however when i use 'BAPI_INB_DELIVERY_CONFIRM_DEC' or 'BAPI_OUTB_DELIVERY_CONFIRM_DEC' i get an error related to serial number assignment, I have passed the serial number as well.