Hey amccrimmon,
It's great seeing you use the power of QuickBooks Online to simplify complex elements of your accounting. The reporting for holdbacks (or retainages) can be very specific, so the ideal way to record these depends on your particular business and accounting method. The system comes with powerful accounts receivable reporting built-in, which can be used to your advantage by making multiple invoices with different terms. If you'd benefit from more advanced payment term options, you can help us enhance the QuickBooks experience by submitting feedback directly to our engineers.
Your accountant is your best resource for help with this. You can easily invite your accountant as a user to approach this together. To search for a pro in your area, simply head to the My Accountant tab in QuickBooks Online then click on Find a pro to help.
Let me know how you end up recording this!