Can I set different pricing for a specific End User?

In order to set specific pricing conditions for a specific user, go to Manage > Users tab and click on the user email of the client you want to adjust the pricing for. 

Once you access to the user profile, click on the "Pricing" tab.

On the following screen you can set user-specific prices. Once you have defined the prices, click "Save user-specific prices" to save the adjusted pricing for the user. 

Is it possible to assign different prices to different releases from the same user?

The pricing is set per account and not per release. Therefore, to modify the distribution pricing for a specific release it has to be done from the user's account, affecting all new releases as a whole.