With Purchase Order Processing it is quite common for the stock to be invoiced before receiving it, therefore in this type of example once the stock is received it becomes part of the overall stock system. If a mistake has been made, or some goods are faulty, it is difficult to reverse the effect that stock has already had on the system. A POP Return can be raised for the relevant quantity, but if your costing method is FIFO, it’s going to use the oldest stock on your system when despatching back to the Supplier, potentially creating temporary imbalances between Stock and Nominal ledger.