I can steer you in the right direction for recording this kind of transaction. Essentially, you have a situation where your customer has become somewhat of a vendor/supplier since they sold product to you. In QuickBooks, customer and vendor accounts are handled separately. This means there isn't a way to apply an amount received from your customer selling you something to their open invoice. Because of that, it takes a bit of accounting maneuvering to work this out. Luckily, we have awesome accountant users here in community who have offered their advice on these kinds of questions previously.
You'll note that I've linked to a QuickBooks Desktop question, but the accounting methodology should be the same. If you have an accountant, I recommend adding them as an accountant user to your QuickBooks Online so they can take a look at your books and help you get set up with this. Learn more here: Invite or remove an accountant
If you're looking for an accountant, QuickBooks can help you there, too! Simply click the My Accountant tab in your account, then the Find a Pro to Help button. From there, you'll be taken to a search where you can use your postal code to find an accountant near you.
I hope that helps! Don't be afraid to stop by community again. :)
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