Negative Qty Cost Tier Question

We have a few Lot items that have the below scenario that we are hoping to clean up:

Item has 2 cost tiers.  Both share the same lot number, but have different Receipt Nos/Dates. 1  has a qty of 490, the other has -490.  I was hoping that I could simply post an Inventory Adjustment Transaction to balance them out, but, while I was able to adjust a positive 490 to the negative tier, I was unable to apply -490 to the other tier, I get 'the distibution quanitty exceeds the available quantity'.  (Even though item maintenace shows a qty of 490 available on the cost detail tab).  Anyone have any suggestions for this?  I'm messing around in a test company so I'm going to try a physical count on the item to get rid of the remaining 490 on hand.  Thanks in advance.