Adding discounts on an invoice is not an unheard of practice. Plenty of law firms add discounts on invoices all the time to nudge clients into motivating clients to make early invoice payment, or reward them for bringing in large amounts of projects needing legal services. Imagine how much more your law firm's cashflow can benefit from that.
To add a discount to an a existing, unpaid invoice
1. Click Invoices
2. Choose the invoice you want to add a discount to
3. Click Edit on the extreme right corner
4. Scroll down to locate and click Discount, then input a discount value and type
5. Close the dialog box and save the invoice