こちらがレターの文章の例ですのでご参考ください。
To Whom It May Concern
This is to confirm that (あなたの名前) has been appointed to (会社名) effective (駐在開始日). His Duration of stay in the United States is ( 駐在年数 ) years. His current gross annual earnings are $ (年収). If you have any questions with retard to this matter, please do not hesitate to contact the undersigned at your convenience.
Truly yours,