Please help to check it

除咗grammer有少少嘅問題外..................應該無咩problem

but I would write like this:

This is an authorisation letter about the sale of the property (Address) on (date). If you are unable to come back to Hong Kong before (date), we'll transfer this document to (name) to sign the document. Please sign and return all documents to the office

很好, 不過若可以加上 confirm 的字眼會較 fornal, 另外最好加上一句倘若收信人沒有簽回確認信, 會被視為同意上述之安排.

Please sign for confirmation and ......... to the office on or before (DATE).
If we do not received your confirmation before the said date, you will be assumed that agree with the above arrangement(s).

這是一封授權信,關於將在(日期)完成買賣(地址)合約授權書,若你不能在(日期)前回香港簽署合將.我們將會用此文件給(名)去代簽署一齊. 請簽署後寄回公司

This is the authorization letter in regarding to the transaction of the property (Address), which will be completed on 11 June 2006. In case of your inability to return to Hong Kong on or before 11 June, we could present this document to (Name) to sign all of the document on your behalf. Please sign it and return it to the office.