Negative stock value showing???

Dear friends

I have one clarification in tally.

Example:Puchase 10 mobiles @ 800 each -8000Rs.

& Sold 10 mobiles @ 1000 each. - 10000Rs.

via Gateway of Tally->Accounting Vouchers->F8>Sales ... updated the sale entry save it.

- I updated the delivery note- via Gateway of Tally->Accounting Vouchers>Alt+F8 

mentioned the same qty 10 & without mentioned the price. Saved it.

My question is while taking stock report ,the mobile closing balance shows negative balance

Purchase   10 qty

Sales           10 + 10 qty  20 qty.(includes sales + delivery note qty)

cl.bal           -10 qty it shows.

But i sold only 10 qty. How to rectify this.

Please guide me. Its very urgent

Thanks in advance

malar.d