Perhaps it’s an "old" customer or one who is key for your studio. Whatever the reason - here is a comprehensive guide to granting them a discount. 

1.Search for the desired customer and open his customer profile
2. Click on the button "Sell product" in the customer profile
3. Select product, date from which the product should be valid, & payment method *
4.Click on "Confirm".
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​5. Click on the product that the customer has just bought.
6. By clicking on product details on the right side a new dialog box will open.
7. Change the payment amount and add a note stating the reason for the discount.
8.Click "Confirm" to save your changes.
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*ATTENTION: Choose a payment method like "Pay later" so that the product is not directly marked as paid and you can still edit the product price.