I've already consulted a lawyer and I just need to pay him a retainer fee, so if I don't get my money back for the operation, I will file a claim for the damages I suffered.
この場合、次のような言い方ができますよ。
ーI've already consulted a lawyer and I just need to pay him a retainer fee, so if I don't get my money back for the operation, I will file a claim for the damages I suffered.
「すでに弁護士に相談は済んでいてあとは着手金を払うだけなので、手術代を返してもらえないなら私が被った損害賠償を請求します」
to consult a lawyer「弁護士に相談する」
retainer fee「着手金」」
to file a claim for the damages「損害賠償を請求する」
ご参考まで！
カナダ 
