we have booked sales return also into that line item 500 pcs , now system should allow us to do again PGI or availabilty for 500 Pcs qty .
When you return the stock,system usually keep the stock in return stock type, after quality check if the stock is okay to re sales then the same stock should be kept in un restricted stock type then system will allow to do PGI again for that qty. You can check the current status of 500qtyu in MMBE.