PO net price to be constant till the invoice-even if there is change in PIR

Hi All,

When a PO is created for material and the net price of the PO is 'X'. when the net price value is changed in the purchase info record to ' X-1' . The invoice document corresponding to the PO would be 'X-1' . Please let me know, what configuration setting needs to be done for the invoice document to have 'X' price.

Regards,

PSS