negative stock levels

I am using "Stock by Attributes" v4.7a on ZC 1.3.8.

We have a number of shoes sizes for a particular shoe style and we allow people to pass through the checkout process when a particular size is out of stock, so it goes on back order. This then removes the qty ordered from the stock by attribute number i.e. goes more negative.

If the TOTAL stock level for a particular shoe style goes negative because a large number of sizes are on back order, then any sizes that actually are in stock also now show out of stock in the cart.

I need to show when a product is out of stock based only on the stock by attribute and not if total goes 0 or negative. Is this a bug and/or can it be fixed?

I have taken a look at the files but can't find where the problem lies. Also, can't find a thread relating to this specific problem.

Thanks,
Dan