Answer:
1) Debit Account Receivable/Bank Account - Sales Amount
Credit Sales Account - Sales amount
Credit Inventory Account - Cost of the product
Debit Cost of Sales  - Cost of the product
2) Debit Sales Account - Sales Amount
Credit Cost of Sales - cost of the product 
Credit Account Receivable/ Bank - Refund Amount 
Debit Inventory Account- Cost of the product 
3) 
a)Any increase in returns over estimate 
 Debit Sales Account - Difference in sales Value
Credit Cost of Sales -  Difference in cost of the product 
Credit Account Receivable/ Bank -  Difference Refund Amount 
Debit Inventory Account- Difference in Cost of the product 
b) Any Decrease in returns over estimate
Debit Sales Account - Decrease in Sales Amount
Credit Cost of Sales - Decrease in cost of the product 
Credit Account Receivable/ Bank - Decrease in refund Amount 
Debit Inventory Account- Decrease Cost of the product 
 
 
Explanation: