I can give you an option...
But you have to take care of the all accounting entries...
At first find the GR material document number...
Display it in MIGO..
Then note the quantity, material, plant, sloc, batch number etc all details..
Then from MB1C, do 561 movement type for the same quantity and same date as per GR date...
Note the 561 movement type's material document number..
Then cancel the goods receipt..
Then you can change the Purchase order condition (Freight amount), Then do GR again...
And after the GR, you can see there are stock in warehouse..
Now cancel the 561 movement types(Or you can do 562 movement type) document number...
Now your stock will be decreased...
And the accounting entry will be reversed for 561 movement type...
Take care of the GR date and initial stock date..