Free goods but want to freight cost updation upon GR

Hi,

Scenario:

I have a PO with 20 line items and header freight cost.

There are few more items in PO which are ticked as free goods.

There is a stock of those materials with certain MAP.

Freight condition has been configured as vendor GR so that it gets loaded on materials upon GR.

Requirement is that

Upon GR of this PO,MAPshould get updated with the freight cost for those materials also which have been marked as free goods.

How this can be achieved ?

Regards,

manOO