Unable to remove the last inventory items

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Hi,

I'm trying to remove the last 2 items of a inventory stock item. When I try to post, I get this message "The full value of stock must be credited when all stock is being removed."

In the Inventory Adjustments & Transfers moduel, I didn't change the amount under the Unit Cost ( $ 20.79) or Amount ($ - 41.58) columns. 

In the Inventory & Services module for this item, the Quantity shows 2 and Total Value is $ 41.58. 

Any advise? 

Thanks