There are two options for entering a discount on an Invoice or Credit Note.
Add a negative line item
Create the invoice or credit note in the normal manner. Add a line item with a negative Unit Price to represent the discount. Please note that the overall value of the transaction cannot be negative.
Adjust the Unit Price
For example, if your Unit Price is usually 10.00 per unit and you wanted to give a 10% discount, then you could enter the Description as "Services (including 10% discount)" and adjust the Unit Price to 9.00.
The same options apply to Purchase Invoices and Credit Notes.