If you want to give a discount to one specific customer, you can do that directly when selling the product.
Follow these steps:
First select the product that you wish to sell. Here we show how to sell a product.
Then, click on the green pencil symbol.
A pop-up appears where you can edit all the settings of this product:
After modifying all the necessary settings, continue the sale by choosing the payment method.
Read here more info about how to sell a product.
In the invoice overview, you will see a a yellow discount symbol meaning that a discount was given for this product.