Manually Adjust Average Cost

Hi, 

We have a product that we purchase 1000 units of and sell individually. The purchase was entered as 1 and we sell it at 1000. For some reason (even though we have multiple products with the same setup), one particular product was using the purchase price for each item we sold. This meant that the box of 1000 for $40 had a cost of $4000 when we sold 1000 of it. I cannot see any difference with this product and all the others with the exact same setup.

Now I can simply adjust the stock to show the correct average cost, however a huge negative $ value now shows in our profit and loss associated to inventory adjustments. Is there anyway at all to correct the average cost without putting it to an account?