Hello Parmanand,
As per my understanding what your trying to do is charge customer with the extra amount which is in addition to the invoice amount. The amount in the supplementary invoice will be known at a later stage than creation of invoice, hope I am understanding it correct.
You want to rasie this supplementary invoice only in case of price difference between the actual price and invoice price.
If my understanding is correct then you should use invoice correction request were in you can just correct the price without any quantity posting as suggested by Lakshmipathi sir.
Please try and let us know if it works
Regards
Naveen K