Can I ask my older client letter (Not employer) saying that this is for future employment (Job offer)?

I am on OPT pending status and I am going to apply H1B for this year April 1st 2016. I worked on my internship (CPT) with my employer for a Client (My job ends on Sep-2015)

So now can I request a client letter from my older client (Not my employer) who I was worked with them, saying that this is for future employment (Job offer)? Can I do that?

Conclusion: My Employer is asking his client for a letter for me for future employment, hope this is make sense. Please help me. Appreciated your help in advance .

Not sure if I understood your question correctly.

If any other company wants to be the eventual employer, then they should be the one filing the H-1 petition.

If any other company wants to be the eventual client, then they need to show a contract w/ your H-1 petitioner and the location mentioned in LCA should be same as the client site.