I'm here to guide you on how to cancel or refund the payment of your customer.
If you're referring to cancel the payment through Merchant services, you can go to the AllSales tab in the Salesmenu find the transaction, and select Void. Please note that you can void the transaction if it's not yet batched or if the payment wasn't processed yet. I've added a screenshot for your additional reference:
However, if the payment has been processed you can use the Refund receipt option in the + New button. I'll show you how:
Go to the + New button, select Refund receipt.
Enter the customer's name.
In the Payment method.
If you're going to refund a sales receipt:
To credit back their credit card, choose QuickBooks Credit - Credit card.
To credit back their bank account, choose QuickBooks Credit - Bank.
If it's an invoice, you can choose among the three options if it's cash, check, or store credit.
In Refund From section, choose which payment method and account you wish to refund the customer from.
Select Save and close.
I've attached a screenshot to see how it looks like:
I've included this article that will help you track and get the details of your business transactions: Run reports in QuickBooks Online. This also contains what you can do more with your reports such as advance customization and specific payroll reports you can pull up.
I'll be here if you have other questions. Let me know in the comment section below. Take care.