Final answer:
The correct journal entry in this case is to debit Merchandise Inventory and credit Accounts Payable by the amount of the allowance, which is $900.
Step-by-step explanation:
The correct journal entry to record the allowance would be $900 debit to Merchandise Inventory and $900 credit to Accounts Payable. When a company receives defective inventory and is given an allowance, this allowance reduces the amount owed to the supplier. Since the question indicates the perpetual inventory system is in use, the merchandise inventory account needs to be reduced by the amount of the allowance. Therefore, the correct entry is to debit Merchandise Inventory, which decreases the asset, and to credit Accounts Payable, which decreases the liability. This transaction reflects the reduction in cost of inventory due to the received allowance, prior to paying the invoice.