Save a client-specific discount so it can be applied when that client's invoice is updated.
Steps
Open the client/contact for editing and open its settings.
Find Invoice Discount Settings and turn on Apply Discount.
Enter the discount name, calculation type, amount or percent, and applicable tax choice.
Wait for the change to save, then open the invoice you are working on and review its calculated discount and total.
Check the current invoice
The contact settings say discounts apply when an invoice is updated. Do not assume saving the default immediately rewrites every old invoice. Check the invoice you intend to change before sending a revised amount to the client.