Hi there carlahay,
It's vital that your customers have the correct balance so that your books can be represented accurately. QuickBooks Online is a great tool able to help you create and send invoices to your customers all while having them entered in your books. I can point you in the right direction for assistance
In order to move the money in between two customers, you could possibly use a transaction such as a journal entry. To know how you should be making this adjusting entry, I recommend reaching out to your accountant, as they'll be able to provide some assistance with how this should be entered to reflect correctly in your books and in the customer accounts. You can invite your accountant to your QuickBooks account so that they can view your books by navigating to the My Accountant tab. You can also find an accounting professional near you by clicking on the Find a pro to help button in that section.
If you have any other questions, feel free to reach out here.