How should I record a transaction where customer invoice paid in full but brings back 1 product which at the same time buys another product which makes refund less?

I’m here to help you record this customer refund in QuickBooks,12314612151149.

Recording a refund to your customer will actually return their payments. If the customer buys another product, you'll need to create an invoice for that item. Allow me to share some steps on how to achieve this in your QuickBooks Online (QBO) account:

1. Go to the + New menu.
2. Click the Refund receipt.
3. Select the customer you want to refund from the Customer drop-down.
4. Fill out the fields using the info from the original sale.
5. Choose which payment method and account you wish to refund the customer from 
6. Then select Save and close.

Once you've recorded the refund, you can then link the refund to the customer's credit. Check out Record a customer refund in QuickBooks Online for more details. 

You can visit this link for more details about the process: Void or refund customer payments in QuickBooks Online.

Let me know if you need anything else concerning the customer refund. I'm always around to help you.