Only tax on profit

[Lilleklump]

Hello!

 

We are having an problem with the tax system in Prestashop. We only want the tax to be on the profit. How do we change that on the panel? 

 

So if we purchase an product for 100 and the sells it for 150 we only want the tax to be 25% of the profit.

 

An example:

(150 - 100) * 0.2 = 10

 

Do we need to modify an module or can we edit this in the admin panel?