Keep FreshBooks for the billing record
FreshBooks should stay where invoices, clients, payments, projects, and time-based billing are managed. That is where the billing record belongs.
FreshBooks already exposes OAuth 2.0 authentication, API resources, and webhook callbacks. The operational work usually starts after one of those billing events matters to a real person.
The admin work around the invoice
Someone still has to notice that an invoice is still in draft, that a payment did not land, that a client needs a reminder, or that approved work should become a bill now instead of next week. That is where receivables and freelancer cash flow slow down.
A practical way to use both
Keep the billing system in FreshBooks. Let Neudash react to the FreshBooks event that matters and handle the reminder cadence, acknowledgment, exception routing, or owner task that follows.
This connection depends on the access FreshBooks already exposes through OAuth 2.0, REST APIs, and supported webhook callbacks.