There are many ways in which this can happen.
For example:
the item was used as material in a production order, and has been fully utilized.
However, this order was later removed.
In this case, the material is automatically returned to the stock.
If you really do not have such a product in stock, then do the following:
1. Restore the item using the Restore button.
2. Write off this amount.
3. Delete the item again.