Items quantity error

1. When I create a bill, the quantity of the item gets increased but if I delete the bill, quantity doesn't get reduced to the previous number.

2. If I make a draft bill, the item quantity is increased to the quantity I entered. But If I delete the draft bill, it doesn't reduce the quantity BUT it also increases the quantity. Suppose, I made a draft bill of an item of quantity 50. When I create draft, main item quantity increases by 50. I f I delete the bill, again the quantity gets increased by 50 more!!!

How to reduce the item quantity?