Hi Zi,
Its quite possible when you don't have quantity but value for the inventory.
Can you confirm which valuation method you are following and extract the detailed transactions level report (Inventory value report) for this product?
Having positive value without quantity indicates that we had positive inventory at higher cost than what we issued.
Example:
Product purchased at 100 , 1 QTY
but when it is sold , it went out at -95 , 1 QTY
now net we have is Zero QTY with 5 as value.
So, now next investigation point can be , generate detailed transactions list for this product and see why issue price was less ?
Also, try running inventory recalculation if applicable.