He tried to pass a bill that would cut tax rates for people who earn over 30 million yen a year which would be an advantage to him.
この場合、次のような言い方ができますよ。
ーHe tried to pass a bill that would cut tax rates for people who earn over 30 million yen a year which would be an advantage to him.
「彼は年収3000万円以上稼ぐ人の課税率を減らす、彼にとって有利になる法案を通そうとした」
to pass a bill「法案を通す」
to cut tax rates「課税率を下げる」
to be an advantage to him「彼に有利になる」
to earn over 30 million yen「3,000万円以上稼ぐ」
ご参考まで！
カナダ 
