ContractorTools can't update the invoice in QuickBooks because payment(s) have been applied to the payment in QuickBooks. The "Invoice balance” will show an amount that is less than the "Invoice amount”, which means that payment has been applied to the invoice.

If you need to change the invoice in ContractorTools, simply delete the corresponding invoice in QuickBooks, and then run the QuickBooks Sync again. Note that this will un-apply the payment to the invoice in QuickBooks, so you will need to re-apply it in QuickBooks.