In Expense Management, Invoice Request allow rate and quantity to be edited. Price x Quantity is business 101.
This will help requestors ensure their data entry is correct and allow purchasers to know the exact quantity without having to look for notes or the attachments. This is a very basic function.
Rates can change on a daily basis and to create expense categories to match every rate is unrealistic.
This should be possible if your expense category is set to calculate by rate.