Customer credit this for items, journal use cogs current average cost

We have transaction as below:
1. Purchase items 1 from Supplier A 2 Unit = @$100
2. Selling items 1 to Cust A  1 Unit = $120
COGS Items 1 is $100

3. Purchase items 1 from Supplier B 1 Unit = @$120
COGS Items 1 is now $110

4. Cust A refund items $120,
GL input as stocks deduct $110, instead $100

as the customer using cogs value before is $100, this should impact for valuation report?

thanks