Sorry Sebastian, I'm not sure I follow - the manager in that flag is a specific user for each requested_for user; it's taken from the 'Manager' field of the user's information (as in the screen Administration > Users > [the details of the user in question]). If you specify a group, instead, then it will go to the group, as you've seen.
Is it that you only wish the policy to fire if the manager is in that group? What are you looking to have happen if that manager user is not in the group you're interested in?