How do I record timesheet invoices generated in Zoho Workerly against a Sales Order?

How do I record timesheet invoices generated in Zoho Workerly against a Sales Order?

We have customers who issue us a Purchase Order for an aggregate amount of hourly services, which we invoice against on a weekly, bi-weekly, or monthly basis (contract dependent).

For simplicity, let's say the customer PO is for $50,000 (1,000 hours at $50/hour) to be invoiced weekly - this is recorded in Zoho Books as a Sales Order. Each week we would generate an invoice for 40 hours * $50 = $2,000 using a timesheet recorded in Zoho Workerly. How do I associate those invoices to the Sales Order so that we can track and report on the balance remaining from the original customer PO?