Fully or partially paid POS order is made. Then cashier may convert it to invoice either by depositing balance (in case of partial paid order) or without any cost and then pilfer the stock. Need confirmation.
Currently we will only come to know that there is a fraud when original customer comes and complains that he did not receive product and at that point we have no way of accepting/ disproving this. Other option is taking a delivery acceptance on order print and saving it for future reference as attachment.