incorrect price on the invoice

can anybody please help me with this. We have a PO for a material which has 38 USD per 100KG and there were 900000KG ordered for this material. But now the invoice was received for partial quntity for about 600000 KG amounting to an amount which comes to 33USD Per 100KG. now what is the way to fix all this. The invoice has not been paid

thanks

Anusha