Hello, vitanovabookkeeping. 

Let's go back to square one so we can trace where the issue is coming from. A negative balance is caused by excessive credit memos and other transactions that affect the Accounts Receivable account (deposits and journal entries). 

Before we create any other transactions, I'm more than happy to work with you and dig a little bit deeper to find out the cause of the negative balance. 

You'll want to check the customer's profile and review the transactions under the Transactions List page. Look for Unapplied Payment or open Credit memos. 

If there are such transactions, there might be an invoice that was deleted. In this case, recreate it with the correct details then apply the credit memo as a "payment" to it. The Audit log can help you trace the deleted invoice. 

To effectively make use of the Audit log: 

1. Go to the Gear icon, then select Audit log. 
2. Click the Filter button.
3. Check Transactions.
4. Under the Show drop-down menu, select Deleted/Voided Transactions.
5. Click Apply, then look for the deleted invoice. 
6. Click the View link to see original invoice details (before being deleted). 

Also, if there are transactions that were recorded incorrectly, you can adjust the amounts or delete them (if necessary). 

If there are any other things you need to take care of in your client's books, feel free to visit our articles from the articles list page. 

Do you have more questions about managing your customer transactions? Lay down the details below and I'll work with you again. I'm also open to any other concerns you might have.