How to knock off Invoice Price Varience a/c

Hi I have maked the PO having an item of Price 10 rupees and Quanity of this Item is 10 therefore po is of 100 and approved it too. Now while doing the Invoicing so I have found that supplier send me the Item with the Price of 8 rupess therefore i have maked the Invoice and Validate it too the 20 goes into the Invoice Price Varience A/c. Now the Question is that how to knock iff IPV a/c ??
What is the Process to do that ??

Regards
Breath