Split products in cart when product is sale product + has stock

Hi, We have 'sale products' in our Magento store. WHEN a product is on stock (quantity is > 0) and attribute 'sale product' is Yes THAN we have a script that automatically puts this product in a sale category and adds a 'sale price' of 20% less than the normale price . Now here is the problem: When a customer puts a product in their shopping cart 20 times with only 2 on stock, they can buy 20 at the sale price. We give far to much discount. We need a fix for this, for example: WHEN a customer buys 20 items of a product that has 2 on stock THAN in the shopping cart should look like: 2 x product to sale price 18 x product to normal price Can someone fix this for us? Extra information: - The stock is not managed by the 'standard' stock function in magento but by an attribute called 'werkelijke_voorraad' (real stock), we enter the amounts ourselves. - The sale items are managed by the yes/no attribute 'saleproduct' - The auto sale function is triggered every night by '[login to view URL]' - In the cart a message is shown like: 'we only have 2 products of this item in stock, the other 18 have a delivery time of 3 weeks' Please respond! thanks.