Accounts Receivable departments often need to model or show discounts on invoices. This help article explains some of the ways to model this and what things you should take into consideration.
If you are writing off bad debt, see Client does not pay invoice or underpays invoice.
If you are trying to refund money from a previous invoice, see Creating Credit Memos.
If you are trying to show a discount on your invoice, continue reading.
Negative Milestone
Negative Soft Cost
Negative Expense