How to track RMA process, costing impact and accounting impact in sales order??

Case 1 (Sales order): I have created an sales order with item A of 1 qty unit item cost of 100 with customer selling  price of 200 on 01-APR-2018.

Revenue : Selling price 200-Item cost 100=100(Revenue on sale).

In this case of on-hand quantity of item A  is decease

On 01-JUN-2018  unit item cost of item A increase from 100 to 150.

Qty    item A (Unit cost)

9        100

10      150

Total cost : 9*100+150*10= 2400

Case 2  (RMA with Referring the original sale order ):  I have created the RMA by referencing the above sales order number with item A return amount of 200 .

In this case on-hand quantity of item A  will increase and it will update qty of