Muhammad,

And agree on a specification for the work. This all sound "legalistic" but you need to mutually define; "what am i to do"; " when am i done?", "When and How do you pay me?"

If the client wants to expand the work you modify the statement (specification) of work.

I think you should tie payments to milestones. The milestones are specific achievement correlated to the specification of work. The 1st payment should be upon delivering an agreed upon specification.

regards,
p