In the Payment tab, click on the invoices bar, click on Credit, enter the discount amount, and click confirm.
Please note: you need to have the appropriate permissions to perform this action.
Alternative route
There is also an alternative route: use the green ‘edit invoice’ link in the Payment tab and apply the discount from there.
Both options have their pros and cons.
Advantages of editing the invoice
There is only one invoice, which is also clearer for the patient.
After the change, the discount is also visible in the record. So if an employee looks back later, the correct amount is shown.
You can still create a credit invoice later if needed for any reason, since you cannot create two credit invoices.
Advantages of crediting
You don’t modify an old invoice, which from an accounting perspective is not entirely correct.