Right click on the item in the stock code selection window and select "View On-Order Details" from the popup menu.  Do you see any entries there with negative quantities?  That should tell you where the on-order quantity is coming from.

My apologies. I thought that the on-order detail would show items with a negative quantity, but I was incorrect.  It will only shot items with positive on-order quantities as it was designed for ordering items rather than returning them to your supplier.

In short, you have an entry for that item with a negative quantity on one of your open purchase orders.  If you don't have that many open POs, just open each one and look for that item with a negative quantity.  You can correct the quantity there.