老闆人在外國, 來郵件說想做匯款, 請問英文這樣回他可以嗎 ? I think it must be done by next week when you return to H.K. Because it need your signature .?

Must be 改為 only could be 或 could only be.
Must : 一定要(該句不是下命令，老闆可以不簽啊!)
only could be done/could only be done : 只有(老闆簽名才)能被完成(匯款才能生效)

I emphasized the importance of a formal remittance+signing your signature on(---)by next week, when you return to H.K.
Hence:-I think it must be done by next week when you return to H.K.
Your signature "on the cheque" is needed.