I see the importance of seeing the remaining balance of your invoice, jan552. Let me help you with this matter. 

Since you've mentioned that your customer already paid the portion yet it shows 0 balance, I suggest pulling up a report named A/R Aging Detail report for you to see if there are open transactions with that specific person. 

Here's how: 

1. Go to the Reports menu and click the Vendors & Payables dropdown. 
2. Choose the A/R Aging Detail report. 
3. Press the Customize report. 
4. In the Display tab, under the Columns search bar, enter Name then tick it. Once done, click OK.
5. In the Filter tab, type the Name in the filter search bar and press that. 
6. Click the dropdown above the Active status, 
7. Select the specific customer, and press OK.

You may refer to this article to learn how to create a receive payment as you see the open invoice in the said report so that it will now apply to your invoice transaction: How to use payment links in QuickBooks Desktop. 

Let me know in the comment section if you have other concerns or additional information as you perform the steps above. We're always here to offer further assistance.