Thanks Anaghan,

Option 1 is not feasable because the category is also used to direct the financial transactions in our accountancy and therefor does not offer much flexibility. 

Option 2 we tried and it calculates the discount on the webshop on the checkout page, but it does not show the prices with a discount on the product page. 
We also noticed that in POS the entered discount is shown on all products and not only on the selection we made. 

Hi, you have two options in Odoo that can fit your requirements. You can choose the one that benefits you the most:

1. POS Pricelist: Allows you to set different product pricing based on product category or by selecting specific products.
2. POS Discount & Loyalty: Provides features for applying discounts and offering loyalty programs to customers. here is the option product tag.

Please check the picture below to see the details and let me know if you have any questions.

Hello, one solution maybe is Server action, where modified with python code. In this server action you should search the products with the tags are you interested and modify the field discount.

This solution is easily to programming in odoo with python. You don t need to use a IDE, you con programming in Odoo, in Settings> Technical> Server Action

Code example in comment.