Okay so just to confirm:
1. Customer places order.
2. Opportunity record and with products created and updated.
3. Opportunity becomes closed/won.
4. Rep takes payment inside salesforce.
5. Datasync runs and creates invoice from closed/won opportunity.
6. At the same datasync session, the payment record from salesforce is created in quickbooks AND applied to the invoice (closed/won opportunity) inside Quickbooks only.
7. At the same datasync session, In Quickbooks, the "Current Balance" for that account is reduced as a result of the payment so as a result the datasync will update the "Current Balance" amount in salesforce allowing us to report and view in salesforce.