If you receive a purchase order with a quantity of 0 you cost may change - BUT, only if your quantity on hand is equal to or less than 0. This is due to Cloud Retailer attempting to use the best information possible in lieu of good information. If your quantity on hand has gone negative that means that we cannot accurately match the receipt of the product from a vendor purchase (or transfer) to the sales / current costs of the product and on that basis, we take the last provided cost as a substitute, temporarily at least.
As soon as you legitimately receive the product the costing will retroactively fix itself - even going backward if needed.
For many clients this fact is helpful - it allows them to adjust costing in cases where they don't want to maintain accurate inventory. They simply import costs into a purchase order and receive it with a quantity of zero. It acts as something of a manual change to cost in this situation.