Feature request - Discount field for Billable time module

There doesn’t seem to be a way to manage efficiently discounts for Billable time records which are batch processed to Sales invoices if a user has restricted access to edit Sales invoices.

One way to get around this is to credit a sales invoice and also copy to a new sales invoice to get to edit the sales invoice but this is not a very efficient approach. Besides you have to remember which sales invoices have discount and how much discount. A discount field in the Billable time module could take care of all of this. Also Billable time module has Division field which is used by Sales invoices edit screen, so @lubos why not also a discount field ?