In his previous job he was known for working dishonestly, so the stakes are high by employing him.
ご質問ありがとうございます。
この場合は、「不義理を働く」は英語で「work dishonestly」と言います。
また、「彼は前の職場には不義理を働いたので敷居が高い。」という文章の英訳を知りたいなら、下記の英文通りになります。
In his previous job he was known for working dishonestly, so the stakes are high by employing him.
ご参考になれば幸いです。