payment Issue

We purchase some material say on 01/01/09 at rate of 5 rs/item - 100 items.

GR & IR was made on say 01/02/09. But payment to vendor was not made due to some reasons.

Nw after some period say on 01/04/09 that PO was ammended to rate of 7 rs/item. Nw when account wants to clear the payment for vendor, in vendor balance it shows the actual ammount but in accounting doc it shows the rate of ammendment. So account person gets confused that hw invoice of less ammount was made. So can any one tell hw to sort it and where he can get the actual info.

Thanks