See clip from Attorney Rajiv S. Khanna's conference call video that addresses this question.
There are only two things.
1. “I” the employee has the good faith intention in taking that job. You may never have to join that job but you should have the good faith intention to take up that job when the green card comes through. You may never have to do it but you should have the intention.
2. The employer must have the same good faith intention of hiring you sometime in the future. What is that time within a commercially reasonable period after you get your green card or any time before that?
Let’s say you get your green card approval today the future employer should hire you within 3,4 5 months. That is the commercial reasonable period.
So applying for a green card through a future job - can it be done and the answer is "yes" it is allowed. The employer should have ability to pay wages and that is an important thing. Other than that all that is needed is good faith intention on part of the employer and employee to give and take the job.