Sameer,
This is controlled by Pricing type.
In copy control, (if delivery>billing; then VTFL OR if order>billing; then VTFA), at item level, give pricing type as D
This shall make the pricing flow from proceeding document without any change.
https://help.sap.com/saphelp_erp2004/helpdata/en/dd/560f03545a11d1a7020000e829fd11/content.htm
In this link, look at pricing type G also.
