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！
He tried to pass a bill in his favor to lower the tax rate.
He attempted to push through a bill that would reduce the tax rate for his own benefit.
・Pass a bill「法案を通す」
・In his favor / For his own benefit「自分にとって有利な / 自分の利益のために」
「自分に有利な」を表現するのに適しています。特に in his favor は「彼に味方するような、彼にとって都合の良い」というニュアンスが強く出ます。
カナダ 
アメリカ合衆国 
