Hello Jaco, its very likely that the customer needs to bring some information from the sale.order.line to the purchase.order.line and even more when we're using de MTO route in our product. This is not avaliable in odoo standard because we have several lines in our sales order than can be one line in the purchase order ej:

sale order line 1: t-shirt with the comment "customized with an odoo logo" with a $5 sales price

sale order line 2: t-shirt with the comment "customized with a coca-cola logo" with a $10 sales price

On our purchase order we will have a one and only line:

line 1: 2 t-shirts

Even though we bring our field "sale.order.item" to the purchase order line you'll need to select the sale.order.line manually because this fields are many2many and there's no way that odoo figures out or knows automatically which sale order line we are talking about regarding that we could have 1 purchase line per multiple sales lines.

There's a forum entry that explains a workaround for service type products which have the reordering configuration as "subcontract service" in case you're talking about services. 

Here's the link: https://accounts.odoo.com/es_ES/forum/ayuda-1/how-to-define-the-po-price-from-the-so-v14-and-v15-199906

Hope this information helps you.

Regards,