Threshold Limits / Check on Quantity Ordered & Price Entered on Purchase Order

Is there a way in which the Quantity or Price can be checked or subjected to a threshold limit? This can prevent or reduce keying error entries. Perhaps the Limit on the Purchase Order should be enabled so e.g. If we already have a Max quantity in stock, then we should not be able to enter a new quantity to be purchased, unless their is already a sales order logged against the quantity on Stock, and then if so, that only the difference in quantity can be ordered and within a price range (otherwise requiring additional overriding authorisations).