I solved the problem by my myself. The answer is simple. I just followed the proverb "In the life nothing is created everything is copied" :) and copied a existing code from the redmine as listed below:
<% if User.current.allowed_to?(:view_repository, @project) -%>
With this "if" I check if the user is allowed to view my button inside the project. Is important to note that the :view_repository is the permission which i have put.
Questions about the solution, please feel free to discuss.
Best regards and thanks!