A person you have a relationship for only business

In a piece of software, what would I call a person you have a relationship for only business in real life. He/she is not your friend and you are not working in same place.

For instance, you are freelancer, and someone wants you hire. So what would I call that person?

Client or business associate might do the job.

In the particular case you cite, they are my customer. If I am a salesman, a plumber, a consultant, or a member of many other professions, they are also called my customer. If the roles are reversed, they might be called my contractor.

Colleague is a good alternative, if you do not want to specify the exact nature of the working relationship...in fact, there may not be one at all, just a shared profession or area of expertise/interest.