In your QuickBooks account, you'll need to have an Accounts Receivable (A/R) account that tracks the money that is due to be received or has been received. This should be an account that QuickBooks creates for you. See this QuickBooks information about Accounts Receivable.
When using Commerce Sync to transfer your sales, an invoice is created to represent the sales. The information on this invoice is represented in Accounts Receivable on your Balance Sheet. Commerce Sync then immediately pays the invoice after it is created with the payments tendered in your point of sale or payment system.
For the Commerce Sync Receivables setting, we recommend using the default QuickBooks "Accounts Receivable" and not using any other A/R account.
Here is the setting in the Commerce Sync Dashboard:
The accounts, items, and customers in your Commerce Sync settings are populated by the information Commerce Sync receives from QuickBooks.