I can't believe you/they wouldn't remunerate someone who teaches English for free.
"英語を無料で教えてくれる人に報酬を出さないなんて" は英語で
I can't believe you/they wouldn't remunerate someone who teaches English for free.
といえます。
しかし、 remunerate はあまり使われないと思います。ま、使う人は使うと思いますけど、compensate のほうがもっと使われていると思います。
I can't believe you/they wouldn't compensate someone who teaches English for free.