Is there a way how to to make a discount price for a product in the nexus shop where the very first purchase is discount and all following purchases of the same product then have a regular price ?

There is the “Previous Purchase Discounts“ on the product pricing page, which is meant to make subsequent purchases cheaper. Might also work the other way around. Just try it out. Might not be the best user experience though. Alternatively, just create a coupon for the user’s purchase of just this product, which can be used an unlimited time but just once per user. That way you have the standard price and the reduced price with the one-time coupon.

Thanks for your ideas - i'll check it out. Well the main idea behind this. I want to offer possible online students the first lesson with a reduced price and it they want to continue from then on it's the regular price.