Hi Shriya,
If your PO value is 1000, then GR will also post with 1000. Unless you change the PO value as 1200,the GR wont post with 1200. If you change the PO as 1200 and the material price indicator as 'v', in that case it will post as 1200 to inventory a/c. If the price indicator as 'S', in that case the inventory got posted with 1000 and remaining 200 posted in PPV account.
At invoice posting, the system will derive the PO value from the PO rate * GR qty, if you change the PO rate as 1500 and GR posted with 1000, in that case 500 will got posted to inventory/PPV a/c, if the price indicator as V. Else it posts to PPV.
However, if the PO rate as 1500 and GR posted with 1200, in that case 300 will go to inventory/PPV a/c.
Regards,
Mukthar