H1B amendment withdrawn

Iahve H1B approved with client A till 2020, my company filed amendment with client B since client A project is closed. I started at client B ( on receipt#) but billing is not yet started for me. Now, I got a call from Client A and work. Can I join project with client A immediately after filling amendment withdrawn with client B or what is the process?

Yes, you file the amendment withdrawal, get that receipt and start working with A.