Hi
Run the Inventory Trail balance report for that problematic Inventory item alone and verify the balances. If you couldn't find the issue then execute the In Integrity Check for that Inventory Item and Site combination with the following Options
1) Validate Inventory
2) Rebuild Inventory Quantities and Cost
3) Rebuild on Order Quantities
4) Rebuild Inventory Plan
After the In-Integrity process still if the issue persists then install the hot-fixes for Quantity based issues using the below link
mbs.microsoft.com/.../SLHotFixList
Hope this Helps
Thanks & Regards