Restriction of Negative Stock ?

Hi,
 
In the Inventory Issues (P31113) program, we have enabled in the Processing Option "1" - Negative On Hand Quantity on the EDIT Tab.
We used make inventory normal issue transactions or reversals on already issued quantity on various Work Orders.
Now our problem is, when some one issued the quantity from the available lot, then stock goes in the Negative (-ve) side.
 
For example :- Item X - Lot Number 101 : +20KG & Lot Number 102 : -20KG
Now on hand quantity is Zero for the above Item X.
But some one can able to issue the material from the Lot Number 101. Then my hand on quantity becomes Negative (-ve).