It is not possible to link a sales order to a purchase order. However, when you convert a sales order into a purchase order, Zoho Books will record that in the Comments & History section.
To check if a sales order has been converted into a purchase order:
Here, you will be able to find a comment “Purchase order raised for Sales order”.
To check if a purchase order was created by converting a sales order:
Here, you will be able to find a comment “Purchase order created from Sales order”.
Books
Online accounting software
for small businesses.