? 
kalo_chasma is right, you don't need to involve HR from your client, your Manager is enough. this is waht I got from my clinet when I went for H1 visa stamping.................
I wrote the letter,a nd asked my manager to sign below for me. he made some changes to the wordings.
Date: - January 27, 2011
TO WHOM IT MAY BE CONCERN
Sub: Employment Verification
This is to verify that Mr. XXXXXXX XXXX (XXX XXXXX XXX) has been working in our IT Department – XXXXX XXXXX, as a consultant, at our location - XXXXXXXXXXXXXXXX, from May 2009 to the present. He is working on a project– XXXXXXXXXXXX as a SQL Server Developer\DBA and Application Analyst\Support. His contract with XXXXXXXXXXX, as a consultant has been extendted recently to the end of the August 2009, with the high possibility of extension in the future.
Mr. XXXXXXXXXX is not an employee of XXXXXXXXX, Inc.
Also, Mr. XXXXX presence at XXXXXXXXX is very important to finish the project in a specified time-period. Currently, he is leading a team of other four consultants, providing information and valuable inputs. We have recently approved his, just over a 2 weeks leave from the work to visit his family back in Nepal and also do his H1-B Visa stamping. He has become an important member in the team for this project, with lots of skills and talents, who always performs his duties\jobs and responsibilities with great effort and on time.
Therefore, we would like to have Mr. XXXXXXXXXX as soon as possible and request to expedite his interview process. For any further inquires and concerns, please feel free to contact us at below –
Thanking you,
XXXXX
Applications Development Mgr.
XXXXXX
XXXXXXX
XXXXXXXXXX
XXXXXXXXXX