delete the validated purchase return

SUGGESTED

The purchase return is validated and its purchase invoice is posted. I would like to ask about how to delete this validated purchase return. I have tried the modification of issues from the purchase return Lines. The modified is in issued STK, which is lesser than the available stk. So an error pop up, 

Application error
The quantity entered is less than the quantity to be booked.

Is there another way to delete this validated purchase return? OR is the step i did for modification of issues in Lines has missed up some steps?