In simple step, the correct process is what ever the GR amount is posted for the PO, you should post Invoice amount as same as GR amount.
In your case, if you find the IR amount is different than GR amount, then you should cancel the GR, then again post the GR after changing the price.
Suppose, you have created a PO with amount 4680, then you have posted GR with amount 4680. But you've got invoice for amount 4320. Now you should cancel the GR and then change the PO amount as 4320 and then again do GR with amount 4320, then do invoice for amount 4320.
Then check your PO history. It will be shown as balanced.
