Based on the information you provided, I suggest checking to see if your new inventory item has an average cost. This can cause negative inventory because you didn't enter a quantity on hand amount; therefore, it affected your average cost amount. You can adjust this by following these steps:
Go to the Inventory tab and select the Inventory Center.
Find the negative item and press it.
In the Transactions box, right-click and hit Edit Transaction.
Under Qty Difference, enter the correct amount.
Choose Save and Close.
Your inventory is now adjusted, and the negative value should've gone away. However, if you keep hitting a bump in the road, I suggest checking out this article on fixing negative inventory issues in Desktop for more information.
Please follow up with me here if the issue persists. I'm determined to help you fix your negative value.