It's vital for all your transactions to be correctly entered so your books can be well balanced. QuickBooks Online is a great tool able to keep track of your sales and expenses to help you see the profitability of your company. I can provide some information about returns.
If you issued a refund to one of your customers, you can create the transaction by clicking on the +New icon, then Refund receipt. If you've given them credit for future purchases instead of a refund, you can select the Credit memo transaction type instead.
If you got a credit for future purchases from a supplier, you can create a Supplier credit by going to the +New menu, then Supplier credit. If your supplier gave you a cash refund or provided a refund to your bank account, you can follow these steps to create a supplier refund, or create a Credit card credit from the +New menu if your supplier reimbursed your credit card.
If you have any other questions, feel free to reach out here.