Ambarish,
I am assuming that already there is a business agreement between your company and the client, that if the goods pass your company's quality inspection, only then the amount shall be credited to the customers.
You can create a credit memo request based on the RE sales order. In the CR memo request, change the quantity from 10 to 8 EA and then create a credit memo.
In the RE order set a billing block. As billing (Credit memo should not be created for this RE order, ever!)