We need to mark excess payment for an invoice, but QB is not allowing for the same

I've got your back, @Pradeep Hari. I'm here to help you record that overpayment without any further delay.

Where did you enter the amount received? You can enter the exact or partial amount in the Payment field. To record the excess, the total amount paid should be entered in the Amount received field.

Here's how to record the overpayment for your customer's invoice:

1. In the upper-right corner, click the Create menu (+).
2. Select Receive Payment.
3. In the Receive Payment window, choose a customer.
4. Under the Outstanding Transactions section, put a checkmark on the invoice you want to pay.
5. In the Amount received field, enter the exact amount you received from the customer (including the overpayment).
   
6. Select Save and close.

This article provides more information on this subject: Record invoice payments in QuickBooks Online.

You can refer to these articles for more insights about handling customers credit or overpayment in QuickBooks Online: 

• Record a customer refund in QuickBooks Online.
• Handle a customer credit or overpayment in QuickBooks Online

Keep in touch with me here should you have any additional questions or concerns about handling customer overpayments. I'm always available to help.