[How-to] link a "Sales Invoice" to a "Project" or "Cost Center"?

I want to use the “Profitability Analysis” Report on based on “Cost Center” and “Projects”.

In order to track all Expenses related I need create a link between “Purchase Orders” and my “Cost Center” or “Project” but (unlike the “Sales Invoice” I can not find any Link field to a “Cost Center”, nor a “Project” in the “Purchase Invoice” Form.

How can I achieve tracking those expenses?

You need to create the link to “Cost Center” and/or “Project” per each single line in the item Subtable of the “Purchase Invoice”. To do so you need to click inside the utmost left field with the item line number which will open a pop-up with the details of the specific item.

Q: Why is this logical?
A: because you might place one single Purchase Order/Invoice of one Supplier for various running Projects and likewise need to be able to assign each order line separately