Once an invoice has been created, you can choose to add a discount product after the fact.
Keep in mind if you are wanting to add a discount to an existing invoice, you will need to make sure you have already created a Discount code and that your discount code has an applicable product within your invoice. Click here to learn more about how to set up Discounts.
How it works
From the invoice page, if there is a line within the invoice that you want to add a discount to, click the 3 dots next to it and press "Add Discount".
From the modal window, there will be a drop down display of all applicable discounts for the product you chose. You will also need to hand enter the amount you want to discount for this invoice. Note that it will not look at the Discount set up and follow what the discount was set up for (percentage or specific dollar amount).
Reversing Discount Added
If you realize you need to remove the discount or adjust the amount that was added to the invoice, you will need to click the "Add Discount" option once again from the main product within the invoice (NOTE: Do not click Reverse next to the discount line as that will create a new invoice to reverse that invoice item).
Make sure you enter in a negative amount in order to correct the discount you originally entered. In this case above, the discount was for $5.00 so if I wanted to correct it and remove that discount, I would need to enter -5.00.