In ideas @Lubos last answer was
Previously, it wasn’t possible to link sales invoice with sales order. Order field on sales invoice was free text field so you’ve put there whatever you wanted.
Now that linking has been implemented and linking provides additional functionality, I need to reconsider this.
I actually understand the process well and the need for customers to trace their order to invoice to be able to compare. So you claim therefore that the Purchase Order needs a reference number that the Seller needs to put on the Sales Invoice to facilitate the Customer in their tracking so that payment can proceed based on the order made and invoice received.
Where we differ is that many businesses including business to business use 3 reference numbers on their invoices:
- Order number
- Customer number
- Invoice number
However, all these numbers are generated by the business. The customer should use these numbers in their own system but you suggest that the business needs to use the customer numbers. So we keep practicing what in my view is more common and you do what you need to do which can already be done with custom fields and custom themes for those that need it.