Comment 1 for bug 586181

Hello Elicoidal,
This is not a bug at all, but just a misunderstanding for you.
The purchase order form will only allow you to select the partners which are Suppliers which means, Company is purchasing some product from the Supplier X.

Once you Create the relevant invoice ,it will be a supplier invoice and the partner(categorised as a supplier for you) X will be treated as the partner for the invoice.

So on the 'Supplier Invoice form' you will see the partner as 'Supplier' , the one who has been selected as a 'Supplier' in the 'Purchase Order form'.

Hope this helps.