How should you enter amount received when customer under pays

I've got the steps to help you enter underpaid invoices, brandon.  

When our customers pay less than the invoiced amount but plan to pay off the balance, we can record it as partial payments.    

Here's how:
 

1. Go to the + New button.
2. Under the Customer column, choose Receive payment.
3. From the Customer dropdown, select the name of the customer.
4. In the Amount received field, enter the amount your customer paid.
5. In the Outstanding Transactions section, select the checkbox for the invoice you're recording the payment for.
6. Fill out the other necessary details. 
7. Click Save and close.

Then, we can review the remaining balance for your customers. Follow the steps below: 
 

1. Go to Reports. 
2. On the Find report by name, search for Customer Balance Summary report. 
3. Click Run report. 
4. Review the customer's open balance in the Total column.

Alternatively, if your customer can no longer pay the remaining balance, we can record or write off bad debt. This means a customer owes you money, but you can't collect it.    

Moreover, I've added this article about personalizing sales transactions in QBO: Customize invoices, estimates, and sales receipts in QuickBooks Online. 

Let me know if you have additional questions about managing your customer's payments in QBO. Keep safe! 

@pccpabrandon-yah 

I think @Erika_K 's response missed the point of your question.

If you want to add the balance due as a result of the underpayment on the customer's next invoice, go to Settings, click on Customer form styles.  Find your invoice template, click edit.  Click on Content, click on the middle section (Table), check the "Show on invoice box".  Click Done.  On the customer's next invoice, it will show an account summary at the top of the invoice showing the amount due as a result of the underpayment and the new charges.  Hope that helps.