how do you do po-invoice matching when dealing with a reseller?

Here is my situation.

I do a PO with supplier A (he is the reseller).  Supplier A will send details to an another supplier (IE: Supplier .  So far so good.   the probleme I have is the following:  Supplier B is sending me the invoice..  I will pay Supplier B, but what shoud I do about  the matching of the PO and the payable invoice?  it is not the same supplier?

How do you adress the situation?  outside process?  any Ideas are welcome.

thanks

Martine