Yes, a potential employer can ask for this. Remember: companies don't have to hire people--they voluntarily choose to hire, and they have essentially free rein (but for not discriminating against certain protected categories, such as on the basis of race, religion, age over 40, sex, disability, etc.) in whom to hire and whom to not hire. An employer may review any material it likes and can use anything that is not discriminatory (as described above) to make the decision as to hiring. IF you had relocated or given up another job based on being told you had the new job, that *might* give you a cause of action based on "promissory estoppel"--but otherwise, the prospective employer may do what you describe.