Account determination for entry ..... PRD not possible

Hi experts,

I ve posted an invoive referring to the PO in MIR7. But there was a price differece between PO and invoice. So I corrected the prices as in the invoice. All materials have V price control. When Iwant to save theinvoce I get a following error:

"Account determination for entry 2000 PRD not possible"

We don't want to post price difference account. But sometimes there can be price difference between invoice and PO. How can I do?