Incoming goods price change

There is a basic thing I'm missing on Stock module and this is that it should be possible changing the purchase price just on moment where we are packing incoming goods.
This is, in spain the way most of companies work is like this:
.- Purchase order, invoice from packing. We fix goods prices according supplier prices but sometimes price is changing so much because of market needs that in a few days it has changed.
.- Receiving the incoming goods, supplier fixes the real price he is going to invoice.
.- A few days after receiving goods, we receive the supplier invoice and the invoice can include goods from several orders, several incoming packs. So we can't create the invoice until we receive the real supplier invoice, and we have no way like it is now of changing price on incoming pack, so we have to write a note or something somewhere to remember the price changed from purchase order to incoming pack.

I think there should be a real price and discount fields on incoming pack filled by default with purchase order prices but available to be changed if needed.
Creating the invoice from packing prices should be taken from this new fields and not from purchase order.

I think this simple think would be a real great improvement for a lot of companies.

Thank you very much:

Ana