GR value is lesser than invoice value

Hi Exeperts,

In purchase order histroy the GR value is showing lesser than invoice value.  The invoice is posted with respect to purchase order (not wrt to GR)and for the invoice payment is done and unable to reverse it. Now invoice amount is correct but that is example 1000 but GR value is like80.   User has posted so many GRS documents and reversed as well so price mis match happned.

Now please suggest me how to correct GR value in PO history

Regards

Maria..