How to treat when we generate an invoice to the client but the client already has debit balance in previous payment. So how to adjust in the new invoice ?

Welcome to the Community, accounts258.

A debit balance in the client's account indicates an increase in the Accounts Receivable. You can create a Credit note and apply it to the new invoice to fix the balance.

Here's how you can create the credit notes:

1. Click the (+) New icon.
2. Select Credit Note.
3. Choose the customer name.
4. Enter the Credit Memo Date.
5. Fill in the Service Date, Product/Service, and Amount fields. You may also fill in any other fields (Description, QTY, Rate) if necessary.
6. Click Save and close.
7.  

Then, you can apply it to the new invoice. Follow the steps below to be guided (Sample screenshot is from US version of QBO reason why it's showing Credit Memo, another term for Credit Note):

1. Go to the (+) New icon and choose Invoice.
2. Fill in the necessary information. Then, click Save.
3. Click Receive payment.
   
4. In the Receive Payment window, enter the Payment date, Payment method, Reference no. (if necessary), and the Deposit to account.
5. In the Outstanding Transactions section, make sure that the correct invoice is selected.
6. In the Credits section, make sure the correct credit memo is selected.
   
7. Hit Save and close.
8.  

Learn more about applying credit notes on invoices through this article: Create and apply credit notes or delayed credits in QuickBooks Online.

Also, feel free to visit this link where you can gain more tips and articles for future use: Welcome India customers to Global Community.

If you have other questions in mind, just let me know. I'll be around to help. Wishing you a great day ahead.