Allow entry of negative amounts for discounts, coupons, etc.
Add the ability for the currency amount field to accept negative numbers so we can bill a client for one amount, then show a discount below it on another line.
This is a no brainer and should be as simple as adjusting the validation rules on the amount field. Do it!
The "overall" discount just doesn't cut it when needing to be specific about credits. The ability to add a - value is critical.
Sean Donovan commented
This is also good for adding credit invoices.
Maybe even make it a percentage of the amount to bill clients who have a regular discount.