[FIX]: check if the product to invoice is the same as sale product

If the product to invoice is not the same product as that on the sale
order, than recalculate the product price.

This is needed for example for the phantom/set bom use case.

Now, if a set/bom product with a price of 100,- is sold, and the
set contains 3 products, the 3 products will be invoiced, each
with a price of 100,-.  This change restores the V7.0 behaviour
which was to recalculate the price.