Glad you're back, @debi8,

Regardless if the sale is fully or partly paid, the steps are similar when it comes to applying a refund. If you're recording the payments manually, I've listed the step-by-step process on how to apply a credit to a partially paid invoice below:

Step 1: Create the invoice.

1. Go to the + New button and choose Invoice.
2. Fill in all the information of the sale and click Save.
3. On the transaction, click Receive payment.
   
4. Record the first partial payment by adding the exact amount received in the Payment field. If it shows the full invoice amount, edit it.
   
5. Hit Save and close once the first payment is received.

Step 2: Create the Credit memo.

1. From the + New icon, choose Credit memo.
2. Enter the customer name to be refunded.
3. In the Product/Service column, pick the item being refunded.
4. Add the correct amount you are going to refund the customer before clicking Save and close.
   

Step 3: Apply the credit to close the invoice.

1. Go back to the + New button and choose Receive payment.
2. In the Customer field, choose the same client name the refund goes to.
3. The result must show the remaining balance of this customer together with the credit memo.
4. Mark the correct invoice number and the credit so the Amount received field shows 0.00.
   
5. Once done, press Save and close.

You can also use the steps outlined in this article as your reference: Create and apply credit memos or delayed credits in QuickBooks Online

If the same behavior happens after following the steps, try logging in to a different browser or private window. Let's check if the problem is brought about by issues in the browser and its cache.

To launch a private window, try out these keyboard shortcuts:

Google Chrome: press Ctrl + Shift + N  
Mozilla Firefox: press Ctrl + Shift + P
Safari: press Command + Shift + N

Clearing the cache can also help resolve unexpected issues in QuickBooks Online.

Let me know how it goes or if you are using a different method of recording the payment. I'll be right here if you need further help with this topic or need anything else in QuickBooks. Have a nice day!