We have the option to convert a Sales Order to a Purchase Order, but without any record of this occurring. Other inventory systems I have used will show full accountability through related transactions.

The Sales Order should be able to link to the relevant Purchase Order, and vice versa so you can easily run through related transactions to a particular order.

This is also true if you have multiple Purchase Orders raised from a single Sales Order so that you can see a full transaction path.

Users who do not have access to Purchase Orders would be able to see the numerical value only but not access the page.