If you have workflows on for invoices it only insists you have the invoice approved if the quantity on the invoice is higher than the receipt quantity. This should also be true for any difference in unit cost as currently you can have a receipt for 1 x £100 and the invoice for 1 x £10,000 and it will not ask for an approval.

It would also be a good enhancement if tolerances are introduced as a maximum value and maximum % and the customer can decide if they wish to have one or both turned on. Rules on these tolerances would be useful for example as long as only one is breached then this is acceptable, or if one is breached then this is not acceptable for example.