Start with the evidence
Check whether the customer supplied a purchase-order or project reference and where they expect it to appear. Preserve the reference exactly, including leading zeros, rather than converting it into an ordinary number.
Work through the example
An order labelled PO-00428 should remain PO-00428 on the draft. Matching it to the customer, scope and currency is a stronger check than finding the same sequence of digits in an unrelated email.
Make the next step clear
Record who confirmed the reference and keep the order with the source material. If the customer changes it, retain the explanation. Do not create an order number just to fill an empty box in a template.
Keep the source context
Treat an invoice draft as a description of an agreed business event. Keep the source agreement nearby and distinguish document preparation from approval, sending and receiving payment. These are separate steps even when they happen close together.