Legal or not, cutting someones salary won't motivate the person...and if she/he found out, you'd have a nice 'enemy' doing all she/he can to torpedo your carreer.

Going to complain with the boss he/she earns as much etc... guess that this is indeed sounding pitty

but you can plead for your cause, as said above mentioning your xtra responsibility etc... that a difference in wage is a stimulation without mentioning names, making clear something isn't ok...
Diplomacy works better then confrontation in most cases, think about ways to bring the message, and don't try to minimize the other's persons position, that is never good except if that person is doing a bad job