Hi Sarle
As mentioned above by Juan M, you will probably be best off using the CardCode field from OITM against the OINV/INV1 tables to show Sales against the supplier.
However, if the supplier for each sold item could be different, with other words the same item is bought from more than one supplier, then this will be difficult to achieve unless you use FIFO or batches where you can base the supplier on an object other than the default supplier.
Hope this helps clarify.
Kind regards
Peter Juby